Restore civ-specific buildings to the civinfo page

Adds a new class CivSpecific which, when combined with the Structure class,
is then used by the civinfo page to identify buildable structures specific to
a selected civilisation. (The SpecialBuilding class previously used was
removed from templates over half a year ago.)

This should resolve the main concern raised on phab:rP24492.

The civinfo captions have also been updated to match.

Patch by: @Nescio
Accepted by: @s0600204
Differential Revision: https://code.wildfiregames.com/D2733