Change locales completion email to use pontoon REST api

[eviljeff]

Fixes: mozilla/addons#15756

Description

Updates command for checking locales completion to use pontoons REST api rather than the soon to be removed GraphQL api. Functionally the same otherwise.

Context

json fixtures are downloads (it's an unauthenticated endpoint), but with the numbers tweaked to make the tests work as before (or with minimal changes).

Testing

• ./manage.py check_locales_completion_rate.py
• check the email in FakeEmail in django admin/shell
• (you can compare with the output from a checked out master - the email was identical when I looked)

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.