Add Gateway's name to RemoteNickFormat (#501)

In order to support extra use cases we should add the `{GATEWAY}` tag to the `RemoteNickFormat` string which would be replaced by the value of the `name=` field from a gateway's configuration.

This is _very_ useful when you are forwarding, for example, multiple channels from one chat to a single channel on another one (one-way). It will help you identify the source channel of a message on the target chat.

gateway/gateway.go

@@ -374,6 +374,7 @@ func (gw *Gateway) modifyUsername(msg config.Message, dest *bridge.Bridge) strin
 
 	nick = strings.Replace(nick, "{BRIDGE}", br.Name, -1)
 	nick = strings.Replace(nick, "{PROTOCOL}", br.Protocol, -1)
+	nick = strings.Replace(nick, "{GATEWAY}", gw.Name, -1)
 	nick = strings.Replace(nick, "{LABEL}", br.GetString("Label"), -1)
 	nick = strings.Replace(nick, "{NICK}", msg.Username, -1)
 	return nick

matterclient/matterclient.go

@@ -344,7 +344,7 @@ func (m *MMClient) parseActionPost(rmsg *Message) {
 	data := model.PostFromJson(strings.NewReader(rmsg.Raw.Data["post"].(string)))
 	// we don't have the user, refresh the userlist
 	if m.GetUser(data.UserId) == nil {
-		m.log.Infof("User %s is not known, ignoring message %s", data)
+		m.log.Infof("User %s is not known, ignoring message %s", data.UserId, data.Message)
 		return
 	}
 	rmsg.Username = m.GetUserName(data.UserId)