Middleware to warn for unapplied migrations

[atodorov]

Not applying migrations can break upgrades and we've already seen installations out in the wild which have upgraded their docker containers without applying the migrations. This can result in serious data loss going forward if users do not upgrade their installations properly.

Django already contains the base functionality that we need:

./manage.py showmigrations will show empty boxes [ ] in case a migration has not been applied.

It is possible to use call_command (see tcms/core/tests/test_set_domain.py) and then parse the output but preferably we can hunt down a specific function in Django that lists all of the migrations and can reuse it instead of relying on text parsing.

Then all of that should be bundled in a middleware, similar to CheckSettingsMiddleware (or even inside of it) and if there are unapplied migrations add warning to the request. It will be displayed in the browser.

Definition of Done:

• Middleware implemented, preferably without using call_command. If this isn't possible directly I need explanation & justification in the PR discussion how Django counts the migrations and which are applied/which aren't and why it won't be possible to use Django's internals for what we need.
• Tests added for the new middleware
• CI pass
• All code review passed & PR merged

Hints:

• Devel getting started docs: https://kiwitcms.readthedocs.io/en/latest/contribution.html#testing
• Django testing documentation: https://docs.djangoproject.com/en/3.0/topics/testing/
• Python unittest documentation: https://docs.python.org/3.7/library/unittest.html
• Check django.core.management.base.check_migrations()

This issue is part of Kiwi TCMS open source bounty program. For more information see the link(s) in bounty-program milestone