R1:ab06a9681c26

Fix two issues with user Calendar event availability cache display

Summary:
Ref T11816. Two minor issues:

- We used `$event`, not `$next_event`, as the event providing the PHID for "Busy at <event name>". This rendered "Busy at <most future event>" on the profile instead of "Busy at <next upcoming event".
- The TTL computation used the event start, not the event end, so we could end up rebuilding the cache too often for users busy at an event.

Test Plan:
- Attended an event in the near future and…