Fix rollback offered when nothing is pushed

Instant Rollback could run when no production deployment was pushed to any station, pausing auto-push on what was really a first push.

• Fixed the rollback dialog to disable Rollback and point to Push to All Stations when no production deployment is currently pushed
• Added a server check rejecting rollbacks with nothing to roll back from, so the procedure can no longer end up incorrectly pinned with auto-push paused