WebMessage: Alert users by email

* Added the feature to receive webmessage notification via email,
  when a new webmessage is sent, both on global level (with the
  configuration variable CFG_WEBMESSAGE_ENABLE_MAIL_NOTIFICATION)
  and on a per user level with the user preference key
  webmessage_mail_notification. (closes #1049)

Signed-off-by: Avraam Tsantekidis <makistsantekidis@gmail.com>