Add new hook AbortTalkPageEmailNotification

This hook will allow extension to disable the regular talk page email notification,
Echo is doing this with a hackish solution of setting global variable
wgEnotifUserTalk to false on the fly, this approach is problematic and will
be replaced with this hook

Change-Id: Ie043d9a1a771d64e4b05b7ddeca75c7542a6c575