Schedule nextup slides generation needs rework

[deram]

Current behavior:

• Events are dropped from nextup slide after 15min from their start time.
• Exception for this is the last event of the schedule, which is never cleared, and slide remains public.

Proposed behavior:

• Add "Now playing" event on top of the list with start time dropped. Now playing event is replaced when event end time is reached or next event has started.
• Add customizable message for schedule, to be displayed on nextup slides when all events have ended. Alternatively just unpublish the empty nextup slide automatically.

These can also be added as user selectable alternatives, as the current behavior is actually working for schedule containing multiple locations mixed up. The proposed behavior would be better for schedule with only one location and events never overlap.