FEATURE: Mark All as Read button for Notifications page

Added a Mark All as Read button to the top/bottom of the notifications user page
https://meta.discourse.org/t/possibility-to-selectively-or-completely-mark-notifications-as-read/20227

Remove notifications property (no longer used)
Don't use __container__ instead use needs: ['user-notifications'], (thanks eviltrout!)
Merge pull request #2874 from cpradio/clear-notifications

FEATURE: Mark All as Read button for Notifications page