Add an optional 'stable' build parameter to mark a build as stable

• Follow-up from the discussion starting roughly here.

This issue proposes to add stable as an optional build parameter, e.g.

Build:1.2.3,123
    commit=v1.2.3
    gradle=yes
    stable=yes

The current implementation abuses CVC for two different things: is-current and is-stable.

If the CVC moves, the is-stable state of the previous build is lost, which leads to several complications.

If this information were kept, it would then be possible to extend the archive logic to be able to express "I want at most 3 stable versions and at most 1 beta of this app in the repository".

It would also make the process of "move-this-to-archive-or-not" much easier (as per #385 (closed)), as the flag could be checked directly.