Merge commit '40b7b8b8f77676', fixes #1937

Merge commit '40b7b8b8f77676', fixes #1937

* commit '40b7b8b8f77676':
  Some improvements to invasion (allow team based invasion if map specifies, allow more monsters to spawn at the same time, precache monsters only if they're used)
  Some minor improvements to monsters (auto precache default model, use monster kills as main point limit in invasion)
  Add a monsters mapinfo feature
  Fix invasion monster spawn counts, fix spawning random monsters, clean up monster spawn system to no longer abuse target_spawn_edit_entity, allow monsters to chase targets through warpzones, fix shambler stomping while in the air, add a new "invincible" spawnflag which prevents monster from taking damage, allow dragging monsters around in edit mode and fix monster count off when monster is gibbed