middleware: implement maintenance mode middleware (bug 1927149)

[zzzeid]

• remove legacy hook code
• add db fixture to some tests
• move dockerflow urls to own namespaced module
• add ConfigurationVariable to admin UI
• return value of variables in get method instead of returning full object
• add maintenance page template
• add maintenance mode middleware, filter out excluded namespaces, urls
• hide login/logout buttons when in maintenance mode
• add maintenance command
• check for pending migrations before enabling maintenance mode (default)

[shtrom]

I wonder if the migrations should be a separate command, that toggles the maintenance mode as needed. This way we could turn on maintenance mode for any other reason where we might not want to add the weight of migrations.

[shtrom]

Alternatively, we could provide a flag such as --skip-migration to achieve the same thing.

[zzzeid]

That was kind of the intention of the force parameter, though I do see your point. Maybe maintenance should modify the flags regardless, and instead we can create pre-deploy and post-deploy commands which can do the migration check instead.

[shtrom]

Where does db come from? Is it a fixture? I can't find its definition (not that it impacts the tests).

[zzzeid]

Good point. This is actually a fixture provided by pytest-django, though after going over the documentation again, it seems it is intended to be used in other fixtures, and not as a shortcut/alternative to marking the test with django_db.

Though it works as expected, I think it's better to explicitly mark those tests as needing the db. I'll file a bug to fix other occurrences in the future, but will change these in this PR.