* Make the ordering of the message and the close button logical on
the DOM.
* Hide the times HTML entity
* Add sr-only label on the close button for non-sighted users
* Add tooltip on the close button sighted users
* With all the notification templates being the same except for the
class, I created a base notification template (core/notification_base)
that alert notifications just override. This avoids code duplication
and can make maintenance simpler.
