Implement OAuth2 provider

[raphaelm]

• Application management
  • Link
  • Tests
• Authorize flow
  • Tests
• Refresh token handling
  • Tests
• Revocation endpoint
  • Tests
  • Mitigate: Refreshing a revoked access token throws an error 500 jazzband/django-oauth-toolkit#585
• API authenticator / permission driver
  • Test
• Enforce organizer restriction
  • Tests
• Enforce scope restriction
  • Tests
• Show current applications to user
  • Revoke
  • Tests
• Log new authorizations
  • notify user
• Ensure other grant types are not available
• Documentation
• check if revoking access toking, then refreshing gets rid of organizer constraint
• Show logentry foo

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@850b9e5). Click here to learn what that means.
The diff coverage is 85.31%.

@@            Coverage Diff            @@
##             master     #927   +/-   ##
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  Coverage          ?   81.42%           
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  Files             ?      246           
  Lines             ?    22308           
  Branches          ?        0           
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  Hits              ?    18165           
  Misses            ?     4143           
  Partials          ?        0

Impacted Files	Coverage Δ
src/pretix/api/views/event.py	100% <ø> (ø)
src/pretix/control/logdisplay.py	31.88% <ø> (ø)
src/pretix/api/views/item.py	97.47% <ø> (ø)
src/pretix/api/views/voucher.py	100% <ø> (ø)
src/pretix/api/views/checkin.py	96.49% <ø> (ø)
src/pretix/base/models/log.py	74.68% <100%> (ø)
src/pretix/api/views/organizer.py	94.11% <100%> (ø)
src/pretix/api/views/oauth.py	100% <100%> (ø)
src/pretix/base/models/waitinglist.py	93.65% <100%> (ø)
src/pretix/control/views/dashboards.py	86.52% <100%> (ø)
... and 10 more