get_app_upgrade_info() improves upon its predecessor by returning more

information about an app. Along with the preferred upgrade method, it

also returns a list of any migrations that are known to have been

applied to the app, and information on whether there are evolutions

and/or migrations available for the app.
Signature generation makes use of this to store the applied migrations

from the database, and an upcoming change for the evolver to support

migrations will use this to best determine the order of operations for

apps that contain both.

Unit tests pass on Django 1.6 and 1.11.