[SCHEMA] Improve timestamp storage

Avoid the use of deprecated MariaDB "zero dates" globally. If they're present
as attribute defaults somewhere, they will be replaced with NULL implicitly.
The existing "zero dates" in MariaDB storage will be left intact and this
should not present any issues.

The "timestamp" type in table definitions now corresponds to DATETIME in
MariaDB with "DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP", which
should be close enough to the original behaviour for compatibility purposes.
It is now the recommended type for "modified" attributes, because of the
update trigger on MariaDB. But there is no such trigger implemented on
PostgreSQL as of this moment.

plugins/EmailSummary/classes/Email_summary_status.php

@@ -20,21 +20,19 @@
  * @category  Data
  * @package   GNUsocial
  * @author    Evan Prodromou <evan@status.net>
- * @copyright 2010, StatusNet, Inc.
+ * @copyright 2010 StatusNet, Inc.
  * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
  */
 
 defined('GNUSOCIAL') || die();
 
-require_once INSTALLDIR . '/classes/Memcached_DataObject.php';
-
 /**
  * Data class for email summaries
  *
  * Email summary information for users
  *
  * @category  Action
- * @copyright 2010, StatusNet, Inc.
+ * @copyright 2010 StatusNet, Inc.
  * @license   https://www.gnu.org/licenses/agpl.html GNU AGPL v3 or later
  *
  * @see       DB_DataObject
@@ -55,7 +53,7 @@ class Email_summary_status extends Managed_DataObject
                 'user_id' => array('type' => 'int', 'not null' => true, 'description' => 'user id'),
                 'send_summary' => array('type' => 'bool', 'default' => true, 'not null' => true, 'description' => 'whether to send a summary or not'),
                 'last_summary_id' => array('type' => 'int', 'description' => 'last summary id'),
-                'created' => array('type' => 'datetime', 'not null' => true, 'description' => 'date this record was created'),
+                'created' => array('type' => 'datetime', 'description' => 'date this record was created'),
                 'modified' => array('type' => 'timestamp', 'not null' => true, 'description' => 'date this record was modified'),
             ),
             'primary key' => array('user_id'),