Query whitelisted emails at authentication time (fixes #500)

[erwa]

Before, the list of whitelisted emails was only calculated once at start-up time, and every time you added a user, you would need to restart Cardboard before that user could log in.

Now we just query the list of emails added to the Google Drive as part of the authentication flow. So as soon as you add a user to a Google Drive, they should be able to login to Cardboard.

The Google Drive API to get the folder permissions is pretty fast, so it doesn't noticeably slow down the login. Also, according to https://support.google.com/a/answer/10445916?hl=en, the default Drive API quota is 10,000 calls every 100 seconds, which is plenty for us.

My implementation of auth_allowed is based off the default auth_allowed implementation here:

• https://github.com/python-social-auth/social-core/blob/911957bc2c0cf0f2e3419e1f1e7caaf25059dc6d/social_core/pipeline/social_auth.py#L12-L14
• https://github.com/python-social-auth/social-core/blob/911957bc2c0cf0f2e3419e1f1e7caaf25059dc6d/social_core/backends/base.py#L148-L159

[codecov[bot]]

Codecov Report

Merging #520 (b4f441e) into master (980274d) will decrease coverage by 0.01%. The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   87.20%   87.18%   -0.02%     
==========================================
  Files         111      111              
  Lines        2532     2537       +5     
==========================================
+ Hits         2208     2212       +4     
- Misses        324      325       +1     

Impacted Files	Coverage Δ
google_api_lib/sync_tasks.py	37.14% <57.14%> (+2.66%)	:arrow_up:
cardboard/settings.py	90.21% <83.33%> (+2.04%)	:arrow_up:

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[erwa]

My first attempt was to run a periodic task on Celery that updated the email whitelisted every minute or so, but then I found out Django doesn't like it if you update settings at runtime -- apparently they're supposed to be immutable: https://stackoverflow.com/questions/6528723/changing-django-settings-at-runtime

I think this approach is easier and cleaner!