Introduce timestamp for plan publishing and use for color coding

• 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