[WIP] Remove applications.Application (bug 1068591) RELOADED

Fixes [bug 1068591](https://bugzilla.mozilla.org/show_bug.cgi?id=1068591)

This is based of PR #285 and takes off from there to fill in for the few little
things missing.
Are you really [sure](https://github.com/mozilla/olympia/blob/master/apps/api/views.py#L356) about [that](https://github.com/mozilla/olympia/blob/master/apps/api/views_drf.py#L203)?