Move complex view condition to a model method

This is moved to a model method rather than an helper method because the
API will need it too.

app/models/note.rb

@@ -358,6 +358,10 @@ class Note < ActiveRecord::Base
     !system? && !is_award
   end
+  def cross_reference_not_visible_for?(user)
+    cross_reference? && referenced_mentionables(user).empty?
+  end
+
   # Checks if note is an award added as a comment
   #
   # If note is an award, this method sets is_award to true

app/views/projects/notes/_notes.html.haml

@@ -2,11 +2,14 @@
   - @discussions.each do |discussion_notes|
     - note = discussion_notes.first
     - if note_for_main_target?(note)
+      - next if note.cross_reference_not_visible_for?(current_user)
+
       = render discussion_notes
     - else
       = render 'projects/notes/discussion', discussion_notes: discussion_notes
 - else
   - @notes.each do |note|
     - next unless note.author
-    - next if note.cross_reference? && note.referenced_mentionables(current_user).empty?
+    - next if note.cross_reference_not_visible_for?(current_user)
+
     = render note