Add configuration example and docs for apache

doc/tls-proxy-apache.md

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+## Apache TLS Proxy for Overleaf
+If you intent to run Overleaf behind the host's let's encrypt TLS apache proxy, you're in the right place!
+
+If you are not using Let's Encrypt but still want to run your Overleaf instance behind apache, have a look at the appendix (bottom of this file).
+
+### Overleaf setup
+In this guide, we assume that overleaf is already running on your host in HTTP, without the included NGINX TLS proxy.
+
+Since we will be running overleaf behind a proxy, we have to change a couple of options in our configuration files.
+
+In your ```config/variables.env```, please ensure that the options are set as below.
+Make sure to change overleaf.example.com to your domain.
+```env
+OVERLEAF_BEHIND_PROXY=true
+OVERLEAF_SECURE_COOKIE=true
+
+OVERLEAF_SITE_URL=https://overleaf.example.com
+```
+
+In your ```config/overleaf.rc```, make sure that the options are set as below :
+```env
+OVERLEAF_LISTEN_IP=127.0.0.1
+OVERLEAF_LISTEN_PORT=8080
+NGINX_ENABLED=false
+```
+Note that the listen port can be changed, but will have to be changed in the apache configuration file.
+
+
+If needed, reinitialize your docker containers with the commands below.
+**WARNING : this will remove your tex libraries in the sharelatex container.**
+
+Refer to [this page](https://github.com/storca/toolkit/blob/master/doc/ce-upgrading-texlive.md#saving-your-changes) if you want to save your TeX install. Redis, mongo data and your documents will still be here.
+
+```sh
+bin/docker-compose down
+bin/up
+```
+
+### Apache setup
+For the apache setup, we assume that you already obtained a valid certificate from Let's Encrypt for your domain.
+
+The apache configuration file is based on a VirtualHost. This file can be found in ```lib/config-seed/apache-le-ssl.conf```. 
+
+**Make sure to change :**
+
+* the ```ServerName``` to your domain name
+* the **port** if you changed it
+* the **path to your TLS private key and certificate.**
+
+The following modules are required :
+```
+mod_ssl
+mod_rewrite
+mod_proxy
+```
+
+#### Redirecting HTTP to HTTPS
+If needed, you can use the following configuration for your HTTP virtual host. Make sure to change your domain name and the port that you've set in your ```config/overleaf.rc```.
+```conf
+# HTTP to HTTPS redirection
+<VirtualHost *:80>
+    ServerName overleaf.example.com
+    RewriteEngine On
+    RewriteCond %{REQUEST_URI} !^/.well-known/acme-challenge/
+    RewriteRule ^.*$ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,QSA,L]
+</VirtualHost>
+```
+
+Run ```systemctl reload apache2``` and you should be good to go!
+
+## Appendix
+### TLDR - just give me the options that work for websockets and secure cookies
+```conf
+RewriteEngine On
+RewriteCond %{HTTP:Upgrade} =websocket [NC]
+RewriteRule /(.*)           ws://127.0.0.1:8080/$1 [P,L]
+RewriteCond %{HTTP:Upgrade} !=websocket [NC]
+RewriteRule /(.*)           http://127.0.0.1:8080/$1 [P,L]
+
+<IfModule mod_proxy.c>
+    # There is no ProxyPass directive ! It is handled by the second RewriteCond
+    ProxyPassReverse / http://127.0.0.1:8080/
+</IfModule>
+
+<Location />
+    ProxyAddHeaders On
+    RequestHeader set X-Forwarded-Proto "https"
+    RequestHeader set X-Forwarded-For "%{REMOTE_ADDR}s"
+    RequestHeader set Host "%{HTTP_HOST}e"
+</Location>
+
+# To avoid SSL stripping https://en.wikipedia.org/wiki/SSL_stripping#SSL_stripping
+Header always set Strict-Transport-Security "max-age=31536000; includeSubdomains;"
+
+#ServerTokens Prod
+LimitRequestBody 52428800
+```
+
+### For a TLS configuration not using Let's Encrypt
+If you are not using Let's Encrpyt but you already have certificates, you can get a TLS configuration for apache from [Mozilla](https://ssl-config.mozilla.org/). Let's Encrypt configurations are based on those.
+
+Depending on your setup (single host or multi-host) derive the configuration you need from Mozilla's website and **add the configuration in the TLDR statement above** to forward websockets and make secure cookies work.

lib/config-seed/apache-le-ssl.conf

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+# Overleaf let's encrypt reverse proxy configuration file for apache2 webserver
+
+<VirtualHost *:443>
+	# The ServerName directive sets the request scheme, hostname and port that
+	# the server uses to identify itself. This is used when creating
+	# redirection URLs. In the context of virtual hosts, the ServerName
+	# specifies what hostname must appear in the request's Host: header to
+	# match this virtual host. For the default virtual host (this file) this
+	# value is not decisive as it is used as a last resort host regardless.
+	# However, you must set it for any further virtual host explicitly.
+	ServerName overleaf.example.com
+
+	ServerAdmin sysadmin@example.com
+	DocumentRoot /var/www/html
+
+	# Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
+	# error, crit, alert, emerg.
+	# It is also possible to configure the loglevel for particular
+	# modules, e.g.
+	#LogLevel info ssl:warn
+
+	ErrorLog ${APACHE_LOG_DIR}/error-overleaf.log
+	CustomLog ${APACHE_LOG_DIR}/access-overleaf.log combined
+
+    RewriteEngine On
+    RewriteCond %{HTTP:Upgrade} =websocket [NC]
+    RewriteRule /(.*)           ws://127.0.0.1:8080/$1 [P,L]
+    RewriteCond %{HTTP:Upgrade} !=websocket [NC]
+    RewriteRule /(.*)           http://127.0.0.1:8080/$1 [P,L]
+
+    <IfModule mod_proxy.c>
+        ProxyPassReverse / http://127.0.0.1:8080/
+    </IfModule>
+
+    <IfModule mod_ssl.c>
+        SSLCertificateFile /etc/letsencrypt/live/overleaf.example.com/fullchain.pem
+        SSLCertificateKeyFile /etc/letsencrypt/live/overleaf.example.com/privkey.pem
+        Include /etc/letsencrypt/options-ssl-apache.conf
+    </IfModule>
+
+
+    # To avoid SSL stripping https://en.wikipedia.org/wiki/SSL_stripping#SSL_stripping
+    Header always set Strict-Transport-Security "max-age=31536000; includeSubdomains;"
+
+    #ServerTokens Prod
+    LimitRequestBody 52428800
+
+    <Location />
+        ProxyAddHeaders On
+        RequestHeader set X-Forwarded-Proto "https"
+        RequestHeader set X-Forwarded-For "%{REMOTE_ADDR}s"
+        RequestHeader set Host "%{HTTP_HOST}e"
+    </Location>
+</VirtualHost>