Server Browser: Allow users to tag servers as "favourites"

Add a way for users to tag one or more game servers as their "favourites". These will be preferred when choosing "play now", even if other servers have more players, so long as they are not full.

Favourite servers will always be placed at the top of the server browser and should have an icon indicating they are a favourite (probably an icon that the user clicks on to toggle the favourite on or off)

This must be saved across sessions (see #101 (closed)) otherwise it's pointless. It may also feed into player count notifications (such as a "favourite servers only" option).