serverRoomList: Enable fixed-height-mode for efficiency

All rows have the same height, so with some minor tweaks we can set
the colum's sizing to fixed and enable fixed-height-mode to speed
up the TreeView.

Fixes #13
1 job for join-room-list in 3 minutes and 52 seconds (queued for 11 minutes and 27 seconds)
Status Name Job ID Coverage
passed build-polari #924