Revive release button on eval page
Don't trigger evaluation of disabled jobsets
Allow setting keepnr to 0
hydra-update-gc-roots: Keep more evals

We now keep *all* unfinished evaluations of a jobset, in addition to
the <keepnr> most recent finished evaluations.

The main motivation is to ensure that mirror-{nixos,nixpkgs} work
properly: if building an evaluation takes too long, some of its builds
may already have been garbage-collected by the time the others finish.
hydra-update-gc-roots: Don't keep the most recent successful view result

Views are deprecated.