Update: new spawn location, safer safe zones, bugfixes

What's new:

• Level layout change: there's a new "shared spawn nest" in the middle with one neutral safe zone.
• Change: players can't kill each other in safe zones anymore by jumping on each other.
• Change: blobs can't kill players inside safe zones anymore.
• Change: mines immediately explode when they bounce into a safe zone (yeah, you know why ;)
• Bugfix: it was possible to pick up a box while in placement mode (eg. if a box fell on the player) which lead to an invalid state.
• Bugfix: it was possible to get killed by a mine in placement mode.