Add spreed.ME reverse proxy to Apache

[oparoz]

We need to connect to the Spreed.ME Snap through a common proxy, so it's best to add it to the one used by Nextcloud. The configuration is available here:
https://github.com/strukturag/nextcloud-spreedme/blob/master/doc/example-config-apache.md

• Add 2 new Apache modules
• Provide configuration

This is to be added either to the default or to the custom config. There shouldn't be any harm in having a location which leads to nowhere.

# Provide access to Spreed.ME
<Location /webrtc>
    ProxyPass http://127.0.0.1:8080/webrtc
    ProxyPassReverse /webrtc
</Location>

<Location /webrtc/ws>
    ProxyPass ws://127.0.0.1:8080/webrtc/ws
</Location>

ProxyVia On
ProxyPreserveHost On
RequestHeader set X-Forwarded-Proto 'https' env=HTTPS

[oparoz]

PR is in #72

[oparoz]

Documentation and test snaps are available here: https://help.nextcloud.com/t/spreed-me-snap-for-the-nextcloud-snap-beta/3757