Newer versions of Django don't have syncdb, and therefore don't have the
post_syncdb signal we use in order to install model baselines and to
determine if we need to recommend installing evolutions.

This change updates our code to listen for a completed migration of an
app when on a newer version of Django. The common bits of code have been
moved into a common method shared by the signal handles for both syncdb
and migrate commands. This also cleans up and documents some of the code
in this path.

Been running this on my local development trees for a while, and tested

it with the Django 1.7+ unit tests in consuming projects.