I added https://github.com/mstriemer/zamboni/commit/79429a2e42784916133f981c06e66e639a0c7664 because when I removed `'reference'` from my `PAYMENT_PROVIDERS` I got an error on this line when I open the management dialog.

Allowing all accounts to be shown even if the provider is inactive requires some more work. I think right now it makes sense to not allow viewing/editing/deleting accounts for inactive providers.

I can add support if you want it though.