R1:e1a9b7694517

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…