Expose CrossReference data via a REST API

Scope

To make CrossReference data usable in scripts, applications, and dashboards (e.g., the Release dashboard), this patch exposes the data through a REST API.

Changes

This change will implement the route cr/api/testfailures/ which provides read-only access to the data via a ViewSet (i.e., ReadOnlyModelViewSet).

It reuses the existing TestFailure model (`class TestFailure(models.Model)`) that is also used to display data on the buildbot.mariadb.org/cr web page.

The select_test_failures function returns a QuerySet that is then serialized to JSON and returned to the client.
Query parameters will be validated using a special serializer APIQueryParamsSerializer.

Examples:

See screenshots attached.