Add a create room button to the UI

The button will be visible on the lobby, and also when in a room (as you
might still want to create a new room while in another room)

Fix #1154.
Hide button when in a room, and fix layout issue when on smaller screens
Display button if room creation is enabled, or if the user is admin
Merge pull request #1165 from JabbR/create-room-button

Add a create room button to the UI