Purchasing with faith

[Thalassicus]

Replace city:IsCanPurchase with the City_CanPurchase function in MT_City.lua

Edit City_CanPurchase to fix these problems:

• [ ] Celtic Abbey can build with faith without beliefs, only for the Celts
• [ ] Jesuits and similar belifs will work
• [ ] can do percent-based national wonder requirements

The function should check the civ can actually build the building type, according to the Civilization_BuildingClassOverrides table.

[GrantSP]

If that function is changed and the extra Celt code is not yet done will the function still work? I'm guessing it should.