Support arbitary event invitees when importing events

Summary:
Ref T10747. When we import a ".ics" file, represent any attendees as simple external references.

For consistency with other areas of the product, I've avoided disclosing email addresses. We'll try to get a real name if we can.

(We store addresses and could expose or use them later, or do some kind of masking junk like "epr...ley@g...l.com" which is utterly impossible to figure out.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16759