Enable django TransactionMiddleware for everything (bug 1076949)

https://bugzilla.mozilla.org/show_bug.cgi?id=1076949

There is little reason to have it only for the API, it's useful everywhere. Nested transactions should work, so we shouldn't need to do any other modifications (hopefully).
If you enable this, how does it deal with [`commit_manually`](https://github.com/mozilla/zamboni/blob/master/mkt/reviewers/views.py#L409)?