Follow-up from "Pass group member data from HAML to Vue"

Convert #members_data_json from a helper to a serializer & entity

The following discussion from !40548 (merged) should be addressed:

• @mayra-cabrera started a discussion: (+3 comments)

  Instead of mapping everything to_json, what do you think if we create a Serializer and Entity for GroupGroupLink? With those in place, we would just need to do something like:

  def linked_groups_data_json(group_links)
    GroupGroupLinkSerializer.new.represent(group_links)
  end

  Serializer and Entity will look like:

  class GroupGroupLinkSerializer < BaseSerializer
    include WithPagination
  
    entity GroupGroupLinkEntity
  end
  
  class GroupLinkEntity < Grape::Entity
    expose :id 
    expose :group, using: GroupEntity
    # ... 
  end