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 subclass to just "add all the patches in this directory", like "autopatches/" works.

Test Plan:
  - Ran `bin/storage status`, saw all normal patches still valid.
  - In some future diff, the instances application will use this to apply patches.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6238

Differential Revision: https://secure.phabricator.com/D10848