Support arbitary event invitees when importing events
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.)
Reviewed By: chad
Maniphest Tasks: T10747
Differential Revision: https://secure.phabricator.com/D16759