Cosmetic: Fix order of imports
Update tower to master tip

The version we had dated back to 2011 and didn't have fixes we made
to tower to better support Django 1.4. This updates it to include
those.
[bug #809449] Fix index name used in tests

This actually does a bunch of things.

1. Fixes the two lines that were hardcoding the index name used to
   "sumo-test" to use the ES_INDEX_PREFIX as well. This allows us
   to put the build number in the ES_INDEX_PREFIX and thus jenkins
   won't use the same index for multiple tests running at the same
   time.

2. Changes a bunch of the test infrastructure to use override_settings
   rather than settings_test.py and mocks.

3. Fixes some users app tests that were interesting.
Cosmetic: Fix pep-8 issues

We import user into the namespace, but then had a bunch of variables
also named user which is bad.
PEP-8 pass on settings.py

Mostly did this to kick jenkins into redoing my tests.
i miss u
Cosmetic: PEP-8 fixes.
[bug 836288] Add on_delete=SET_NULL to last_post FKs.

Otherwise, deleting a user that happens to be the author of last
post CASCADES to deleting the thread and/or forum.