Squad

* core:
* Mark optional fields optional
* fetch project and group for Test
* add DB indices for identifier-like fields
* comparison: avoid extra, slow database query

* api:
* allow write-only access to backend tokens
* add 'email' method to the Build object
* added tests for build/email REST api
* add Group and UserGroup objects to REST api
* add tests for Group REST API
* allow changing slug in UserGroups
* add 'id' and 'metadata' fields to Build object
* add all Project fields to REST API
* allow filtering on most of the Project fields
* add search and sorting features
* ci:
* add reference to resubmitted TestJob
* make `parent_job` read-only in admin
* lava: allow supressing resubmit emails with backend settings
* core: add Group.description
* frontend:
* display list of groups in home page
* add link to TestJob's parent
* display model.name instead of model.slug
* doc:
* install.rst: document installation of RabbitMQ
* restructure docs and include ci setup
* scripts/git-build: allow building package without having runtime dependencies
installed

* 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