Implement OAuth2 provider (#927)

- [x] Application management
  - [x] Link
  - [ ] Tests
- [x] Authorize flow
  - [x] Tests
- [x] Refresh token handling
  - [x] Tests
- [x] Revocation endpoint
  - [x] Tests
  - [x] Mitigate: https://github.com/jazzband/django-oauth-toolkit/issues/585
- [x] API authenticator / permission driver
  - [x] Test
- [x] Enforce organizer restriction
  - [x] Tests
- [x] Enforce scope restriction
  - [x] Tests
- [x] Show current applications to user
  - [x] Revoke
  - [x] Tests
- [x] Log new authorizations
  - [x] notify user
- [x] Ensure other grant types are not available
- [x] Documentation
- [x] check if revoking access toking, then refreshing gets rid of organizer constraint
- [x] Show logentry foo

src/pretix/base/models/log.py

@@ -41,6 +41,7 @@ class LogEntry(models.Model):
     datetime = models.DateTimeField(auto_now_add=True, db_index=True)
     user = models.ForeignKey('User', null=True, blank=True, on_delete=models.PROTECT)
     api_token = models.ForeignKey('TeamAPIToken', null=True, blank=True, on_delete=models.PROTECT)
+    oauth_application = models.ForeignKey('pretixapi.OAuthApplication', null=True, blank=True, on_delete=models.PROTECT)
     event = models.ForeignKey('Event', null=True, blank=True, on_delete=models.SET_NULL)
     action_type = models.CharField(max_length=255)
     data = models.TextField(default='{}')