Implement a workaround to signal Opus support (mumble) (#1764)

* Mumble: Implement a workaround to signal Opus support without pulling in the CGO gopus dependency.

* mumble: lowercase error messages

* mumble: Add link to #1750 in bridge/mumble/codec.go

bridge/mumble/mumble.go

@@ -185,6 +185,7 @@ func (b *Bmumble) doConnect() error {
 		gumbleConfig.Password = password
 	}
 
+	registerNullCodecAsOpus()
 	client, err := gumble.DialWithDialer(new(net.Dialer), b.GetString("Server"), gumbleConfig, &b.tlsConfig)
 	if err != nil {
 		return err