add invite command, enable relay for invites

[maltee1]

Invite a user by typing !signal invite <phone> in a bridged room. Since this is primarily intended for rooms bridged across several platforms for a user of another platform to invite a signal user, I also added relay support for invites. Relaying invites can be disabled in the config. I made the default true, because I believe this is expected functionality. In particular if a non-logged-in user invites a logged-in-user, they don't expect them to be kicked by the bridge bot on the next sync.