Check each throttle class before returning a 429 response in signing view

[diox]

If a single throttling class disallows the request, the other throttling classes never see the request and it never gets added to their history. That means essentially the throttling classes after the first one that failed don't count some requests towards their limit.

I submitted an issue and pull request to DRF fixing this (encode/django-rest-framework#6667), it was merged but hasn't been released yet, so let's backport it.

See also #6655

[EnTeQuAk]

There is a new release that contains this fix out now.

[EnTeQuAk]

Actually, it might not be it yet… mozilla/addons-server#11899 (comment)

[EnTeQuAk]

encode/django-rest-framework#6836 with upstream PR encode/django-rest-framework#6837

[diox]

We just merged mozilla/addons-server#12273 which updates our DRF to 3.10.3, and that should have both my patch and @EnTeQuAk 's fix.