Add ghost migrations warning to runserver

[Jacob Walls]

On the ​forum, several folks indicated interest in supplementing the runserver warning about missing migrations with a warning for "ghost" migrations: migrations recorded on the database but not present on disk, presumably as a consequence of switching away from a feature branch under development.

I suggested wording along the lines of:

The following migrations are recorded as applied but no longer exist in your project:
…
Your project may not work correctly if they are reversible and are intended to be reversed.
To reverse the migrations, recover them (e.g. by switching branches) and run:
“manage.py migrate <app_name> <last_migration_on_disk>”
If you are certain the migrations do not need to be reversed, run:
“manage.py migrate <app_name> --prune” to remove references to them.

[Natalia Bidart]

Hello Jacob, thanks so much for creating this ticket as a follow-up of the forum thread. I have read the posts and while I understand the goal, I'm not sure there is a clear consensus about having this feature. My main concern is not so much about "votes" (no one has objected), but I fear different users have posted favored slightly different features. Specifically, no one replied to your --prune question and I'm very interested in following up that lead.

I'll be closing as wontfix for now but I will comment on the forum to get a clearer idea of what would be the specific feature we would commit to maintain. I'll be happy to reopen and accept once we have more concrete details.

[Natalia Bidart]

Adding some folks for more awareness.