Closed #23365 -- Makemigrations allows timezone-aware datetimes

When a user is adding a new, non-nullable datetime field to their model, 
they're now able to use django util's timezone module to supply a 
timezone-aware value. The prompt also shows an example using the 
preferred timezone.now() instead of datetime's naive now().
@timgraham This looks fine by me, it's actually what I originally had at the DjangoCon sprints, but @freakboy3742 mentioned I should be more explicit about the repr change so that our code doesn't depend on datetime's repr function remaining constant. Ultimately, will still work just fine though and if a new version of python breaks it ever the tests should catch it right away.