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
It's too late for me to start thinking about that this evening, so I'll come back to it tomorrow. I think there are benefits in both approaches: replace it = keep things simple, might work better on smaller screens (mobile view); have both buttons = more options.

I'd suggest posting your question on the original issue, #5113, so that everyone who's posted opinions on that issue will get a notification and be able to give their view. That way we can get more opinions and see if there's a general agreement for one or the other.