qa workflow: Support updating a reference collection with results

For regression tracking (#791), the blueprint requires changing the qa workflow to add a mode where it updates a reference collection with the results of QA tasks.

This involves adding the reference_qa_results, update_qa_results and prefix workflow parameters, as described in the blueprint, and implementing the logic described in Behavior with update_qa_results set to True.

To support generating the timestamp field of QA results, various workflows will also need to know the suite that QA is being run against; so we should rename reverse_dependencies_autopkgtest_suite to qa_suite, with associated data migrations for both workflows and workflow templates.

Ideally, this should also include the proposed on_assign event logic from the blueprint; however, in some sense that's only an optimization, so if that turns out to be too difficult then please file a separate issue for it.

Depends on #905 (closed).