Give organizers in ICS exports a dummy email address to placate Gmail

Summary:
Ref T10747. In the in-email ICS event card that Gmail shows, it has a "Who" field which reads "Unknown Organizer*" if the URI for the organizer isn't email-address-like.

Previously, we used a URI like https://phabricator.install.com/p/username, which I think is OK as far as RFC 5545 is concerned, but Gmail doesn't like it.

Instead, use PHID-USER-asdfa@phabricator.install.com, which doesn't go anywhere, but makes Gmail happy. Users don't normally see this URI anyway.

Test Plan:
Got a readable "Who" in Gmail when importing an event exported from Calendar:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16772