Enable AutoDevOps pipeline features

Now we have a proper CI runner to test with, we can re-enable most of the AutoDevOps pipeline. A couple of jobs are still disabled as documented in the new .gitlab-ci.yml file.

Split out the overrides for running tests and building documentation and move them to a separate template. It's intended that, in the fullness of time, this template moves into a dedicated "CI templates" repo and the "include:local" configuration in ".gitlab-ci.yml" is replaced with an "include:file" configuration which pull it in from the template repo.

The test job runs tests: flake8 and unit tests. The coverage report is submitted and any build artefacts are uploaded.

There is now a documentation job which runs in parallel to the tests and which builds the documentation as an artefact. This is uploaded separately. When we ship GitLab Pages in the Developer Hub, we should be able to browse the documentation directly from the GitLab UI. For the moment, we can download it as a zip file.

Closes #1 (closed), closes #2 (closed)