Add HTML email support to Echo notification

To test the HTML email:

1. install the latest version of php-mail and php-mail-mime package, they are required
   by the core sendmail function to send HTML email
2. set $wgAllowHTMLEmail = true before loading Echo in LocalSetting.php

Change-Id: Ia4b98b14e135742b84f1b0e04589b0efdd24e954
(bug 46665) Add HTML email support to email digest

Change-Id: I3b881acbcf4b18fc0401364ea0a6bc993d2c2246