Squad

* ci:
* move backend settings to database
* ci/lava:
* fix resubmit() function
* properly handle invalid SSH handshake
* core/notification:
* drop test history info from default templates (eliminates a performance
bottleneck with builds that have a large number of failed tests)
* skip creating HTML version of the email if not required
* plugins:
* add testjob postprocessing hook

* celery: correctly load configuration from Django settings

* settings: take Celery broker URL via environment

* fix crash in admin by removing debug print

* api: add REST API for email templates
* ci/lava:
* don't crash if there is no metadata
* update listener to match new LAVA state machine

* core, api: migrate to Django REST Framework tokens
* The old squad-specific Token model has been migrated to Django REST
Framework tokens. The actual data for the old tokens is still there, but
it's not used for anything anymore. It will be automatically removed in the
next release.
* core:
* add support for test case variants
* frontend:
* return 404 on non-existing data
* add initial user settings UI
* add API token settings page
* improve display of descriptions in project list
* api
* Add CORS support via django-cors-headers
* fix TestSerializer
* make Project.slug readonly
* ci/lava:
* add more strings to automatic resubmit list
* parse LAVA log using incremental parser
* add unit test for log parsing