Remove legacy Quake bbox expansion: map entities

This gives QC full control of bboxes
and makes SVQC and CSQC behaviour consistent.

Adds a Small item bbox which is the size most items had previously
if expansion wasn't counted.

Changes powerup, buff and mega pickups to have the same height.
This will make it cheaper to network that information to clients.

Groups all the sv_gameplayfix cvars together in xonotic-server.cfg.

Removes duplication of the item mins/maxs definition
and box + '1 1 1' code.