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
If we're going to change the behaviour of the 'mark all as read' button rather than adding a second button, I'd suggest leaving the text as is. We can announce when releasing this update that the 'mark all as read' button will now mark only those notifications in the current filter, and people will soon get used to the change. So let's wait to see the result of the vote on whether to change the existing button or to add a second button alongside it. If the vote is to add a second button, I'll put more thought into a good label for it.