Allow plumbing an existing Matrix room to a Facebook group chat

[mrjohnson22]

Analogous to mautrix/whatsapp#202.

[JuniorJPDJ]

It probably will need to add support for relaybot, as far as I know facebook bridge doesn't support one.

[rollingmoai]

It probably will need to add support for relaybot, as far as I know facebook bridge doesn't support one.

Related issue: #13

Now that relaybot has been added, this can be worked on now.

[HansJK]

This would be fantastic! Have many existing rooms that would be very annoying to replace.

[gabrc52]

If this is implemented, it would be nice if everyone on the Matrix room is allowed to talk on Facebook, whether or not they are logged in. Perhaps messages on Messenger could come from a designated "bridge" Messenger account, and be prefixed with the matrix username/displayname.

[rollingmoai]

If this is implemented, it would be nice if everyone on the Matrix room is allowed to talk on Facebook, whether or not they are logged in. Perhaps messages on Messenger could come from a designated "bridge" Messenger account, and be prefixed with the matrix username/displayname.

This is already implemented in #13. The issue here is to support plumbing an existing room for relaying.