R1:8e9c20c9ae63

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…