And use :migration tag to use deletion strategy, and to avoid caching tables, and to lock into a particular schema.

Attempting to fix intermittent spec errors `PG::UndefinedTable: ERROR:  relation "public.untracked_files_for_uploads" does not exist`.

Later migrations added fields to the EE DB which were used by factories which were used in these specs.

And in CE on MySQL, a single appearance row is enforced.

The migration and migration specs should not depend on the codebase staying the same.

* Hopefully fixes spec failures in which the table doesn’t exist
* Decouples the background migration from the post-deploy migration, e.g. we could easily run it again even though the table is dropped when finished.