Fixed error when deleting person with gifts.

The gifts were still present on the backbone side, but rendering
them failed. Now when a giver is destroyed, we remove the gifts
from their recipients, and vice versa.
Fixed: can't add recipient right after adding person.

When you added a person, we rerendered the add recipient dropdowns, but
we didn't yet have an ID for the new person, so selecting the new person
would have no effect. Now we wait until we get the ID back from the
server.
Papered over errors in complex deletion scenarios.

Like:

1. Add a new person.
2. Add them as a recipient to two givers.
3. Delete one of the gifts.
4. Delete the new person.

Something wrong with event handlers and relations. Now we just try to
play it safe.