- removes APNS support
- changes the `django-push-notification` library from the `iskhakov`
fork to the [`grafana`
fork](https://github.com/grafana/django-push-notifications). This new
fork basically just patches an issue which affected the database
migrations of this django app (previously the library would not respect
the `USER_MODEL` setting when creating its tables and would instead
reference the `auth_user` table.. which we don't want)
- add `--no-cache` flag to the `make build` command
**NOTE**
A migration should be applied as follows:
```bash
# remove the four push_notifications tables, which have improper foreign key references
python manage.py migrate push_notifications zero
# recreate the tables with the proper foreign key references
python manage.py migrate
```
* move mobile notifications to a separate backend, remove critical notification
* remove outdated mobile app code
* MOBILE_APP_PUSH_NOTIFICATIONS_ENABLED -> FEATURE_MOBILE_APP_INTEGRATION_ENABLED
* create error log if no devices are set up
* move mobile auth related code to the mobile_app Django app
* move mobile auth related code to the mobile_app Django app
* move mobile auth related code to the mobile_app Django app
* fix typing
* add GCMDevice todos
* add user connection capabilities
* add user connect/disconnect to the messaging backend
* move APNS endpoint to mobile_app Django app
* restore critical notifications
* support hackathon app
* tweak migrations so mobile app auth tokens are preserved
* reuse notify_by IDs
* use mobile app template to render push notification
* add GCM/FCM (Android) support
* fix unlink user
* logger.error -> logger.info
