RFE: Improve support for stable-rc

Handling stable-rc in regzbot will always be a special case, as there usually won't be any tags (e.g. Fixes:, Link:, or Closes:) to detect fixes that have been applied between an -rc and the final. So using regzbot for stable-rc might only be worth it, if someone is committed to follow up each tracked issue manually -- or if a CI system semi-automatically does so.

If anyone is willing to do so, then we need a few changes in regzbot as well:

• As of now regzbot will likely struggle to associate stable-rc specific version numbers (like v6.6.21-rc1); that should be easy to fix, as it could just consider them as unknown, but detect that they are from a stable series -- that should be enough for a proper association in the UI (and not that hard).
• Stable-rc commit-ids are volatile, regzbot thus would need to track them by something else (commit summary and author?) to see if a culprit made it into a release or was dropped (definitely not easy)).