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