R1:a1f5fc22318c

Move directory SQL patch construction to abstract base class

Summary:
Ref T6238. I'm building the instance management application now, but not putting it in the upstream -- I think the only use case for it is to build SAAS. If someone comes up with a use case (maybe a college course that wants to create an instance per-class or something?) we could open it up eventually, but it seems cleaner to keep it out of the upstream until we have such a use case.

I need to add schema patches. Make it easier for a…