Django 1.7+ runs our test suite pretty slowly, due to all the migration

work it performs for all the models on every test. We can speed this up

by removing some unneeded settings. We no longer use any middleware

(which shouldn't have an effect, but isn't necessary), and, more

importantly, we no longer include a bunch of Django apps that aren't

really needed.
The app removal is the big thing. The Sites, Auth, and Admin apps are no

longer included, meaning a lot fewer models to write to the database.

This makes a very noticeable impact on the run time. There's only one

test that needed to be changed for this, to check for a different

(mandatory) model in the database signature.
To make this work, though, we needed to make the Session and Auth apps

optional. The builtin evolutions list assumed those would always be

included, and they normally would be, but it's not necessary to always

have them there. We now check for RuntimeErrors, which those will

raise if they're not listed in INSTALLED_APPS, and only add to the

builtin evolution list if we can import successfully.

Unit tests pass on Django 1.6 through 1.10.