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" / "Decline" buttons if any project you're a member of is invited.
• If you're invited, highlight any projects which you're a member of to make that more clear.

On other calendar views:

• If you're invited as part of a project, show the "multiple users" icon.
• If it's just you, continue showing the "add one user" icon.

Test Plan: Viewed month view, day view, detail view. Invited groups and individuals. Invited "Dog Project", accepted invite as user "Dog".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16868