Refactor Capturable Farmstead feature/templates

Attempts to address Invalid warning messages for capturable farmsteads and creates a mixin as suggested by @Langbart

Renames the "farmsteads_" template files to "mill_" to eliminate confusion with standard buildable farmsteads and changes the refs in the skirmish maps to suit.

Genericizes the name to Rotary Mill instead of Farmstead, again to eliminate confusion between the two (the former is a capturable map object, while the latter is a player-buildable structure).

Fixes #6818
Differential revision: https://code.wildfiregames.com/D5113