Squad

* 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

* admin: allow manually triggering post-processing of test run data
* api: add `filter_fields` to BuildViewSet
* api: add ID field to objects returned by REST API
* api: Add ProjectStatus to REST API
* ci: fetch test result data in chunks in LAVA backend
* ci/lava: restore downloading logs
* frontend: Add pagination to test and metric listings
* frontend: fix environment label in tests history table
* frontend: improvements to the resubmit UI
* frontend: paginate "all test results" page
* scripts: add script to migrate testruns between projects
* Store and display metadata about environments, suites, tests, and metrics

* frontend:
* retitle test results page to "All test results"
* hide non-failing test results by default
* improve formatting of "test results" tables
* link test results to test history in "all test results" page
* ci/lava: don't crash when lava listener provides no status

**Upgrade notes:** this version drops support for starting the celery-related
daemons (worker and scheduler) using the "./manage.py | squad-admin" command
line interface. To start those daemons, you should now use the standard celery
command line interface, i.e.

* `celery -A squad worker` for the worker, and
* `celery -A squad beat` for the scheduler.


**Changes:**

* api:
* Added a REST API for accessing most of the data in SQUAD. This API is only
adequate for read-only access for now.
* The API is self-describing and features a API browser when accessed with a
web browser. This API browser is linked from the navigation bar at the top.
* frontend:
* make login link redirect to original page
* settings:
* respect proxy headers when generating absolute URLs
* upgrade to celery v4
* Drop support for SQL-backed periodic task scheduling
* requirements.txt: drop restriction for using celery v3

* core/notification: don't spam admins