Fix mentioned users on diff notes

Summary

DiffNote, and LegacyDiffNote returns empty array for mentionable_attrs, because mentionable_attrs is not inheritable by subclasses. The problem can be illustrated with this small sample:

module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    def attr_mentionable(attr)
      mentionable_attrs << [attr.to_s]
    end

    def mentionable_attrs
      @mentionable_attrs ||= []
    end
  end
end

class A
  include Mentionable

  attr_mentionable :foo
end

class B < A
end

A.mentionable_attrs
=> [["foo", {}]]

B.mentionable_attrs
=> []

Possible solution using cattr_accessor:

module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    def attr_mentionable(attr)
      mentionable_attrs << [attr.to_s]
    end
  end

  included do
    cattr_accessor :mentionable_attrs, instance_accessor: false do
      []
    end
  end
end

class A
  include Mentionable

  attr_mentionable :foo
end

class B < A
end

A.mentionable_attrs
=> [["foo"]]

B.mentionable_attrs
=> [["foo"]]

B.mentionable_attrs < [:bar]
=> [["foo"], ["bar"]]

A.mentionable_attrs
=> [["foo"], ["bar"]]

mentionable_attrs is inheritable by subclasses. If a subclass changes the value then that would also change the value for parent class. Similarly if parent class changes the value then that would change the value of subclasses too.

What are the relevant issue numbers?

Fixes #19807 (closed)

Fixes #18022 (closed)

/cc @stanhu @DouweM @rspeicher