Allow specifying a server to contact for a /join command

Element supports /join !someinternalroomid matrix.org. This can be really nifty and I have used it to recover from wiped databases: you tell your server to join you to the room ID as specified, by contacting matrix.org and getting room info from there. As long as other servers believe you're already in the room or have permission to join the room, you will be properly joined to the room.

This is especially handy when your server forgot that you were in a invite-only room, but other servers still think you are there. You can't rejoin it normally and people can't invite you because other servers don't see why you would invite someone that's already in the room, but by using this method you can properly rejoin it.

Now NeoChat supports /join since cd1bec99, it would be great if it could support this too.