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
I'll think about this. We need something nice and short if it's to fit on a button. My first thought is 'Mark %{type} as read', where `type` = 'reshares', 'likes', 'comments' and so on. If I can think of brief descriptions which work (and which are likely to be easily translatable) for each filter, that's probably a good solution; otherwise, I'll try to come up with a catch-all phrase.