Make it possible to mark certain releases (e.g. unstable/alpha) as "development" releases

Another idea spawning from #311949: Provide us with core developer releases. This is also closely related to #313827: make it possible to not have *any* branch recommended, and in the end, this one might just get marked duplicate if that grows into a larger solution that can cover this, too. But, I wanted to start a separate issue about this aspect of the problem.

It'd be nice if the new "unstable" releases for core didn't show up in the "Official releases" download table, even though they are official releases from a tag, as opposed to automatically-regenerating dev snapshots from the end of a branch. Ideally, they'd show up in the "Development snapshots" download table.

However, I'm not sure:

a) How would you specify this as a project admin at node/N/edit/releases (e.g. http://drupal.org/node/3060/edit/releases)?

b) How would this actually appear in the download table? What terminology would we use to distinguish these from the moving-target tarballs from -dev releases?

Once we had answers to those, there might still be questions about how to actually implement this. ;) But, let's at least start with those two things, and if someone contributes some reasonable mockups, we can look into what it'd take to make it work...