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:

```ruby
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`:

```ruby
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 

Fixes #18022

/cc @stanhu  @DouweM @rspeicher 

See merge request !5243