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/api/oauth.py

@@ -0,0 +1,45 @@
+from datetime import timedelta
+
+from django.utils import timezone
+from oauth2_provider.exceptions import FatalClientError
+from oauth2_provider.oauth2_validators import Grant, OAuth2Validator
+from oauth2_provider.settings import oauth2_settings
+
+
+class Validator(OAuth2Validator):
+
+    def save_authorization_code(self, client_id, code, request, *args, **kwargs):
+        if not getattr(request, 'organizers', None):
+            raise FatalClientError('No organizers selected.')
+
+        expires = timezone.now() + timedelta(
+            seconds=oauth2_settings.AUTHORIZATION_CODE_EXPIRE_SECONDS)
+        g = Grant(application=request.client, user=request.user, code=code["code"],
+                  expires=expires, redirect_uri=request.redirect_uri,
+                  scope=" ".join(request.scopes))
+        g.save()
+        g.organizers.add(*request.organizers.all())
+
+    def validate_code(self, client_id, code, client, request, *args, **kwargs):
+        try:
+            grant = Grant.objects.get(code=code, application=client)
+            if not grant.is_expired():
+                request.scopes = grant.scope.split(" ")
+                request.user = grant.user
+                request.organizers = grant.organizers.all()
+                return True
+            return False
+
+        except Grant.DoesNotExist:
+            return False
+
+    def _create_access_token(self, expires, request, token, source_refresh_token=None):
+        if not getattr(request, 'organizers', None) and not getattr(source_refresh_token, 'access_token'):
+            raise FatalClientError('No organizers selected.')
+        if hasattr(request, 'organizers'):
+            orgs = list(request.organizers.all())
+        else:
+            orgs = list(source_refresh_token.access_token.organizers.all())
+        access_token = super()._create_access_token(expires, request, token, source_refresh_token=None)
+        access_token.organizers.add(*orgs)
+        return access_token