AKPlan/AKModel: Make recently changed colors depended on plan publication

Instead of allowing to edit the colors on an AK by AK basis, as proposed in #138 (closed), visualize changes in the plan only if they occurred after publishing the plan.

This requires the following steps:

• Add a timestamp to store when the plan was published last to event
• Create admin actions to publish and unpublish the plan
• Create link to toggle publication status to edit page of event
• Remove possibility to directly publish the plan from event creation wizard
• Respect that timestamp (and visibility status) in re-coloring method