Added Mark all in filter as read feature

Integrated feature
Added tests
In response to a users request(Issue #5113) i have added a feature that allows users to mark all notifications in the current notfication filter they're on as read without marking all the others read, i have also added tests for this feature in spec/controllers/notifications_controller_spec.rb
A catch-all could be as simple as 'mark all shown as read' (i.e. all notifications currently displayed will be marked as read'). The language is slightly inelegant but it's acceptable. I might be able to improve it; I'll get back to you.