[Core] Add getLastLogin helper to user class

This moves the query to get the last user login from the
dashboard code to the User class, where it more accurately
belongs.

(This was extracted from aces#5896 where it was necessary because
multiple modules use the last login time to determine if something
is new.)

modules/dashboard/php/dashboard.class.inc

@@ -40,15 +40,7 @@ class Dashboard extends \NDB_Form
         $site   = $user->getSiteNames();
 
         $userID     = $user->getUsername();
-        $last_login = $DB->pselectOne(
-            "SELECT MAX(Login_timestamp)
-             FROM user_login_history
-             WHERE Login_timestamp < (SELECT MAX(Login_timestamp)
-                FROM user_login_history
-                WHERE userID=:UserID AND Success='Y')
-             AND userID=:UserID AND Success='Y'",
-            array('UserID' => $userID)
-        );
+        $last_login = $user->getLastLogin();
 
         $siteID = $user->getCenterIDs();
         $this->tpl_data['user_site'] = $siteID;

php/libraries/User.class.inc

@@ -532,5 +532,27 @@ class User extends UserPermissions
             (string) $this->userInfo['Password_hash']
         );
     }
+
+
+    /**
+     * Return the last login of this user to the given database.
+     *
+     * @param \Database $DB The database connection to check
+     *
+     * @return string The date in YYYY-MM-DD format.
+     */
+    function getLastLogin(\Database $DB)
+    {
+        return $DB->pselectOne(
+            "SELECT MAX(Login_timestamp)
+            FROM user_login_history
+            WHERE Login_timestamp <
+                (SELECT MAX(Login_timestamp)
+                    FROM user_login_history
+                    WHERE userID=:UserID AND Success='Y')
+                AND userID=:UserID AND Success='Y'",
+            array('UserID' => $this->getUsername())
+        );
+
 }