Create a supported testing release for power-users

Debian's recommendation against users running testing (due to a lack of security support) means we don't have a reasonable answer for desktop users who want to run reasonably current software.

This hurts Debian's credibility among power-users and probably deprives us of a developer pool that are interested in helping to maintain desktop environments and the experience around them.

Running testing instead of unstable has advantages: It protects users from a large number of developer mistakes, transitions, and other archive inconsistency. There are also disadvantages: Packages in use may be dropped from testing, and security updates may be held-up by transitions.

I see two potential ways out of this:

1. Make unstable more useable.
   • By gating all uploads through another queue that then migrate to unstable ASAP (like Ubuntu's -proposed pocket)
2. Make testing supportable.
   • Maybe by flagging up security issues caught in unstable, and have a team that helps shepherd them through (or upload to testing-proposed-updates, if necessary)
   • Probably: Document mixed testing/unstable, and maybe provide some tooling to help users running this. (e.g. pull security updates from unstable, when necessary)