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 thought it would work, but :

https://docs.djangoproject.com/en/1.6/topics/db/transactions/
> During the deprecation period, it’s possible to use atomic() within autocommit(), commit_on_success() or commit_manually(). However, the reverse is forbidden, because nesting the old decorators / context managers breaks atomicity.

I should probably update the code to use the new `ATOMIC_REQUESTS` database setting and `@atomic` decorator.