Fix hard-to-read text on campaign and training page in frontend at October 31
The text was yellow against a white background (the moon in the Halloween background).
This is fixed by darkening the critical area.
Update outdated and broken holiday stylesheets
Each style was updated by copying qt.css and then adjusting
the required definitions such as background image and color.
Fix resurrection animation appearing at wrong position for some missions and styles
Move max. number of crate spawns into variable
Lobby/room: Prevent opening context menu if no player selected (fixes
bug #661)