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/urls.py

@@ -4,7 +4,9 @@ from django.apps import apps
 from django.conf.urls import include, url
 from rest_framework import routers
 
-from .views import checkin, event, item, order, organizer, voucher, waitinglist
+from .views import (
+    checkin, event, item, oauth, order, organizer, voucher, waitinglist,
+)
 
 router = routers.DefaultRouter()
 router.register(r'organizers', organizer.OrganizerViewSet)
@@ -52,4 +54,7 @@ urlpatterns = [
         include(question_router.urls)),
     url(r'^organizers/(?P<organizer>[^/]+)/events/(?P<event>[^/]+)/checkinlists/(?P<list>[^/]+)/',
         include(checkinlist_router.urls)),
+    url(r"^oauth/authorize$", oauth.AuthorizationView.as_view(), name="authorize"),
+    url(r"^oauth/token$", oauth.TokenView.as_view(), name="token"),
+    url(r"^oauth/revoke_token$", oauth.RevokeTokenView.as_view(), name="revoke-token"),
 ]