Fix comments related to BE part

- Edit commit messge body to fix "danger-review"

app/serializers/merge_request_sidebar_basic_entity.rb

 # frozen_string_literal: true
 
 class MergeRequestSidebarBasicEntity < IssuableSidebarBasicEntity
+  expose :current_user, if: lambda { |_issuable| current_user } do
+    expose :can_merge do |merge_request|
+      merge_request.can_be_merged_by?(current_user)
+    end
+  end
 end
-
-MergeRequestSidebarBasicEntity.prepend_if_ee('EE::MergeRequestSidebarBasicEntity')

ee/app/serializers/ee/merge_request_sidebar_basic_entity.rb

-# frozen_string_literal: true
-
-module EE
-  module MergeRequestSidebarBasicEntity
-    extend ActiveSupport::Concern
-
-    prepended do
-      expose :current_user, if: lambda { |_issuable| current_user } do
-        expose :can_merge do |merge_request|
-          merge_request.can_be_merged_by?(current_user)
-        end
-      end
-    end
-  end
-end

ee/spec/serializers/ee/merge_request_sidebar_basic_entity_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-describe MergeRequestSidebarBasicEntity do
-  let(:project) { create :project, :repository }
-  let(:merge_request) { create(:merge_request, source_project: project, target_project: project) }
-  let(:user) { create(:user) }
-
-  let(:request) { double('request', current_user: user, project: project) }
-
-  let(:entity) { described_class.new(merge_request, request: request).as_json }
-
-  describe '#current_user' do
-    it 'contains attributes related to the current user' do
-      expect(entity[:current_user].keys).to contain_exactly(
-        :id, :name, :username, :state, :avatar_url, :web_url, :todo,
-        :can_edit, :can_move, :can_admin_label, :can_merge
-      )
-    end
-  end
-end

spec/fixtures/api/schemas/entities/merge_request_sidebar.json

@@ -2,6 +2,7 @@
   "type": "object",
   "properties" : {
     "id": { "type": "integer" },
+    "iid": { "type": "integer" },
     "type": { "type": "string" },
     "author_id": { "type": "integer" },
     "project_id": { "type": "integer" },