FIX: Always ensure notifications are treated as read once clicked

UX: improve messaging so notifications list is far more stable
PERF: improve performance of notifcation lookup queries

- Add feature "SetTransientHeader" that allows shipping info to server
   in the next Ajax request
- remove local storage hack used for notifications
- amend lookupStale to return hydrated objects, move logic into store
- stop magically clearing various notifications (likes, invitee accepted, group_summary, granted badge)

spec/controllers/application_controller_spec.rb

@@ -85,6 +85,35 @@ describe TopicsController do
 
   end
 
+  describe 'clear_notifications' do
+    it 'correctly clears notifications if specified via cookie' do
+      notification = Fabricate(:notification)
+      log_in_user(notification.user)
+
+      request.cookies['cn'] = "2828,100,#{notification.id}"
+
+      get :show, {topic_id: 100}
+
+      expect(response.cookies['cn']).to eq nil
+
+      notification.reload
+      expect(notification.read).to eq true
+
+    end
+
+    it 'correctly clears notifications if specified via header' do
+      notification = Fabricate(:notification)
+      log_in_user(notification.user)
+
+      request.headers['Discourse-Clear-Notifications'] = "2828,100,#{notification.id}"
+
+      get :show, {topic_id: 100}
+
+      notification.reload
+      expect(notification.read).to eq true
+    end
+  end
+
   describe 'set_locale' do
     it 'sets the one the user prefers' do
       SiteSetting.stubs(:allow_user_locale).returns(true)