Let entities garrison using cmpGarrisonable.
Followup to r24957 / rP24957.
Moves the logic for garrisoning from cmpGarrisonHolder to the entity that is being garrisoned.
Also fixes https://wildfiregames.com/forum/topic/36863-teleportation-feature-in-a24/?do=findComment&comment=418441 while at it (setting garrisoned before transferring orders on transform).
Differential revision: D3280
Refs: #5906
Comments by: @Stan, @wraitii