Make event invitees behave a little better for stub/ghost events

Summary:
Ref T11326. Currently:

• The month view and day view (ghosts) don't show that you're invited to a child event.
• The detail view copies the invite list, including attending status, but only after it shows the page for the first time.

Instead, for now, just do this:

• Ghosts/stubs use the parent invite list, but treat everyone as "invited".
• Materializing a stub just saves the list as-is (i.e., invited, not a copy of attending/declined/etc).

This behavior may need some refining eventually but is at least reasonable (not obviously bad/buggy).

Test Plan:

• Viewed month/day views, now shown as "invited".
• Viewed detail view, now invitee list shows up properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16758