Introduce a new field that denotes the timestamp when the plan was published
Introduce admin actions to publish and unpublish the plan and set that timestamp accordingly
Prevent other means to change visibility of the plan
Use that timestamp to only highlight recent changes when they occurred after the plan was already published
This implements most of #159 (closed) -- only a link to directly toggle the changing from the change or status view is missing, but that requires !133 (merged) to be merged first
