R1:edba4bb8d471

Improve Calendar event behavior for group invites

Summary:
Ref T11816. Projects can be invited to an event, but the UI is currently fairly agnostic about them.

Instead, introduce the idea of "RSVPs", which are basically invites for you as an individual or for any group you're a part of. When we go to check if you're invited, we check for you individually first, then check for any groups you belong to if you haven't already accepted/declined.

On the calendar detail page:

- Show the quick "Join" /…