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
AFAIK that string is missing in config/locales/diaspora/en.yml. I'd also like to keep "mark all as read" if no filter is active and keep the button seperated from the show all / show unread buttons (keep the indentation).