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:

{F1890571}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

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

src/applications/calendar/storage/PhabricatorCalendarEvent.php

@@ -758,8 +758,18 @@ final class PhabricatorCalendarEvent extends PhabricatorCalendarDAO
 
     $host_handle = $handles[$host_phid];
     $host_name = $host_handle->getFullName();
-    $host_uri = $host_handle->getURI();
-    $host_uri = PhabricatorEnv::getURI($host_uri);
+
+    // NOTE: Gmail shows "Who: Unknown Organizer*" if the organizer URI does
+    // not look like an email address. Use a synthetic address so it shows
+    // the host name instead.
+    $install_uri = PhabricatorEnv::getProductionURI('/');
+    $install_uri = new PhutilURI($install_uri);
+
+    // This should possibly use "metamta.reply-handler-domain" instead, but
+    // we do not currently accept mail for users anyway, and that option may
+    // not be configured.
+    $mail_domain = $install_uri->getDomain();
+    $host_uri = "mailto:{$host_phid}@{$mail_domain}";
 
     $organizer = id(new PhutilCalendarUserNode())
       ->setName($host_name)