convert SubRepoPermsStore to an interface
Created by: camdencheek
This converts the new SubRepoPermsStore into an interface to match
the pattern that the rest of the stores in internal/database
now use.
Additionally, it generates mocks from that interface and adds it to
the database.DB
interface.
@ryanslade, for context, this is some recent refactoring I've been working on that converts the stores to interfaces, generates mocks for them, then replaces the global mocked functions in the stores with non-global injected mocks.