Improve migrations/schema docs
Fixed #22369 -- Added count parameter to assertTemplateUsed
Fixed #22356 -- Added a check to make sure unique_together fields are local.
Refs #18586 - Refactored model get_or_create test.

Refactored get_or_create test into several smaller test functions across two
different test classes. Also converted the comments over to docstrings.
Merge pull request #2553 from liavkoren-vmfarms/ticket_18586

Ticket 18586