Elephant garrisoning should be specified explicitly by the template

[elexis]

Archer and war elephant can only be garrisoned in the elephant stables, but the support worker elephant can be garrisoned nearly everywhere. Duplicate of #3611, but not reopening that.

It should be consistent:

• Only elephant stables should allow garrisoning for now.
• All elephant types (worker, archer, war, hero) can be garrisoned equally. If a building wants to allow garrisoning the support elephant, it should likely allow the other elephant types as well. If it wants to prohibit garrisoning of one type of elephant, it likely wants to prohibit garrisoning of all other elephant types as well. Therefore all GarrisonHolders need either Elephant or !Elephant.

(It's probably not feasible to only add !Elephant to some parent and remove the garrison list from all templates inheriting that.)