Add checks / command options for "migrations not applied" and "migrations needed"

[Daniel Hahler]

The runserver command prints a notice in case migrations are not applied:

def check_migrations(self):

"""
Checks to see if the set of migrations on disk matches the
migrations in the database. Prints a warning if they don't match.
"""
executor = MigrationExecutor(connections[DEFAULT_DB_ALIAS])
plan = executor.migration_plan(executor.loader.graph.leaf_nodes())
if plan:

self.stdout.write(self.style.NOTICE("\nYou have unapplied migrations; your app may not work properly until they are applied."))
self.stdout.write(self.style.NOTICE("Run 'python manage.py migrate' to apply them.\n"))

I would like to use this test during deployment.

Maybe this would be appropriate for the new --deploy option for the check
command introduced in Django 1.8?

Additionally, I'd like to check if there are no migrations required, which I
currently test by looking for "No changes detected" in `makemigrations
--dry-run`.

Even when this doesn't fit into the checks system, there should be a supported
way to get theses information by means of return codes from a command.