Add project id to merge request notable data

app/serializers/merge_request_noteable_entity.rb

@@ -50,6 +50,10 @@ class MergeRequestNoteableEntity < IssuableEntity
     merge_request.project.archived?
   end
 
+  expose :project_id do |merge_request|
+    merge_request.project.id
+  end
+
   expose :archived_project_docs_path, if: -> (merge_request) { merge_request.project.archived? } do |merge_request|
     help_page_path('user/project/settings/index.md', anchor: 'archiving-a-project')
   end

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

@@ -32,7 +32,8 @@
     },
     "is_project_archived": { "type":  "boolean" },
     "locked_discussion_docs_path": { "type":  "string" },
-    "archived_project_docs_path": { "type":  "string" }
+    "archived_project_docs_path": { "type":  "string" },
+    "project_id": { "type": "integer"}
   },
   "additionalProperties": false
 }