Add ability to procure avatars from the destination bridge (#1000)

* remote_avatar: add UseLocalAvatar

* remote_avatar: make sure msg.Protocol is always set correctly

* remote_avatars: support msg.Account

* remote_avatar: add to matterbridge.toml.sample

* remote_avatar: clarify something

gateway/router.go

@@ -132,6 +132,9 @@ func (r *Router) handleReceive() {
 		r.handleEventFailure(&msg)
 		r.handleEventRejoinChannels(&msg)
 
+		// Set message protocol based on the account it came from
+		msg.Protocol = r.getBridge(msg.Account).Protocol
+
 		filesHandled := false
 		for _, gw := range r.Gateways {
 			// record all the message ID's of the different bridges