Rooms links don't open up in RiotX

1. Send a message with a room name and autocomplete it (eg. #hello:example.com). It gets expanded to a link that point to https://permalink.example.com/#/room/#hello:example.com
2. Click on the link
3. It opens up in your browser instead of RiotX

According to https://github.com/vector-im/riotX-android/issues/1113, matrix.to links already open up in RiotX, but that doesn't seem to be the case if you use a custom permalink prefix.