Stop loading MergeRequestEntity data on sidebar request

app/serializers/merge_request_basic_entity.rb

 class MergeRequestBasicEntity < Grape::Entity
+  expose :assignee_id
   expose :merge_status
   expose :merge_error
   expose :state

app/serializers/merge_request_entity.rb

 class MergeRequestEntity < IssuableEntity
   include RequestAwareEntity
 
-  expose :assignee_id
   expose :in_progress_merge_commit_sha
   expose :locked_at
   expose :merge_commit_sha

app/views/shared/issuable/_sidebar.html.haml

@@ -140,7 +140,7 @@
 
     :javascript
       gl.sidebarOptions = {
-        endpoint: "#{issuable_json_path(issuable)}",
+        endpoint: "#{issuable_json_path(issuable)}?basic=true",
         editable: #{can_edit_issuable ? true : false},
         currentUser: #{current_user.to_json(only: [:username, :id, :name], methods: :avatar_url)},
         rootPath: "#{root_path}"

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

@@ -3,7 +3,6 @@
   "properties" : {
     "id": { "type": "integer" },
     "iid": { "type": "integer" },
-    "assignee_id": { "type": ["integer", "null"] },
     "author_id": { "type": "integer" },
     "description": { "type": ["string", "null"] },
     "lock_version": { "type": ["string", "null"] },

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

@@ -8,7 +8,8 @@
     "total_time_spent": { "type": "integer" },
     "human_time_estimate": { "type": ["string", "null"] },
     "human_total_time_spent": { "type": ["string", "null"] },
-    "merge_error": { "type": ["string", "null"] }
+    "merge_error": { "type": ["string", "null"] },
+    "assignee_id": { "type": ["integer", "null"] }
   },
   "additionalProperties": false
 }