Update domain and reverse proxy path

[ChurchOfTheSubgenius]

The Commune server typically runs on a subdomain like api.commune.sh. Now that the client also intends to work as a regular matrix client, it is perhaps better to run the server at a sub path of the actual matrix homeserver domain - matrix.commune.sh/commune/api.

This makes it straightforward to detect whether a homeserver is Commune capable of not. If the commune/api endpoint returns nothing, the client can assume that it's a pure matrix homeserver.