Modernize "mailKey" for Calendar Event

Summary: Ref T13065. Migrate "mailKey" and drop the column.

Test Plan: Ran "bin/storage upgrade", got a clean report, saw migrated data in mail properties table.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13065

Differential Revision: https://secure.phabricator.com/D21633

resources/sql/autopatches/20210316.almanac.13.event-mailkey.php

···
+<?php
+
+$event_table = new PhabricatorCalendarEvent();
+$event_conn = $event_table->establishConnection('w');
+
+$properties_table = new PhabricatorMetaMTAMailProperties();
+$conn = $properties_table->establishConnection('w');
+
+$iterator = new LiskRawMigrationIterator(
+  $event_conn,
+  $event_table->getTableName());
+
+foreach ($iterator as $row) {
+  queryfx(
+    $conn,
+    'INSERT IGNORE INTO %R
+        (objectPHID, mailProperties, dateCreated, dateModified)
+      VALUES
+        (%s, %s, %d, %d)',
+    $properties_table,
+    $row['phid'],
+    phutil_json_encode(
+      array(
+        'mailKey' => $row['mailKey'],
+      )),
+    PhabricatorTime::getNow(),
+    PhabricatorTime::getNow());
+}

resources/sql/autopatches/20210316.almanac.14.event-dropmailkey.sql

···
+ALTER TABLE {$NAMESPACE}_calendar.calendar_event
+  DROP mailKey;

src/applications/calendar/storage/PhabricatorCalendarEvent.php

···
   protected $isCancelled;
   protected $isAllDay;
   protected $icon;
-  protected $mailKey;
   protected $isStub;
 
   protected $isRecurring = 0;
···
   }
 
   public function save() {
-    if (!$this->mailKey) {
-      $this->mailKey = Filesystem::readRandomCharacters(20);
-    }
-
     $import_uid = $this->getImportUID();
     if ($import_uid !== null) {
       $index = PhabricatorHash::digestForIndex($import_uid);
···
         'isCancelled' => 'bool',
         'isAllDay' => 'bool',
         'icon' => 'text32',
-        'mailKey' => 'bytes20',
         'isRecurring' => 'bool',
         'seriesParentPHID' => 'phid?',
         'instanceOfEventPHID' => 'phid?',
···
     ) + parent::getConfiguration();
   }
 
-  public function generatePHID() {
-    return PhabricatorPHID::generateNewPHID(
-      PhabricatorCalendarEventPHIDType::TYPECONST);
+  public function getPHIDType() {
+    return PhabricatorCalendarEventPHIDType::TYPECONST;
   }
 
   public function getMonogram() {