Use cattr_accessor instead duplicating code on NoteOnDiff concern

app/models/concerns/mentionable.rb

@@ -14,14 +14,14 @@ module Mentionable
       attr = attr.to_s
       mentionable_attrs << [attr, options]
     end
-
-    # Accessor for attributes marked mentionable.
-    def mentionable_attrs
-      @mentionable_attrs ||= []
-    end
   end
 
   included do
+    # Accessor for attributes marked mentionable.
+    cattr_accessor :mentionable_attrs, instance_accessor: false do
+      []
+    end
+
     if self < Participable
       participant -> (user, ext) { all_references(user, extractor: ext) }
     end

app/models/concerns/note_on_diff.rb

@@ -4,9 +4,6 @@ module NoteOnDiff
   NUMBER_OF_TRUNCATED_DIFF_LINES = 16
 
   included do
-    attr_mentionable :note, pipeline: :note
-    participant :author
-
     delegate :blob, :highlighted_diff_lines, to: :diff_file, allow_nil: true
   end
 

app/models/concerns/participable.rb

@@ -41,9 +41,12 @@ module Participable
     def participant(attr)
       participant_attrs << attr
     end
+  end
 
-    def participant_attrs
-      @participant_attrs ||= []
+  included do
+    # Accessor for participant attributes.
+    cattr_accessor :participant_attrs, instance_accessor: false do
+      []
     end
   end