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).
I ran some tests and it wasn't necessary, it seems to be working fine. So I vote for not doing that in this PR but rather with the rest of the django 1.7 stuff.