api: add 'system', 'upvote', and 'downvote' fields to note queries

app/models/note.rb

@@ -360,6 +360,10 @@ class Note < ActiveRecord::Base
     create_new_cross_references!(project, author)
   end
 
+  def system?
+    read_attribute(:system)
+  end
+
   def editable?
     !read_attribute(:system)
   end

doc/api/notes.md

@@ -31,7 +31,10 @@ Parameters:
       "state": "active",
       "created_at": "2013-09-30T13:46:01Z"
     },
-    "created_at": "2013-10-02T09:22:45Z"
+    "created_at": "2013-10-02T09:22:45Z",
+    "system": true,
+    "upvote": false,
+    "downvote": false
   },
   {
     "id": 305,
@@ -45,7 +48,10 @@ Parameters:
       "state": "active",
       "created_at": "2013-09-30T13:46:01Z"
     },
-    "created_at": "2013-10-02T09:56:03Z"
+    "created_at": "2013-10-02T09:56:03Z",
+    "system": false,
+    "upvote": false,
+    "downvote": false
   }
 ]
 ```

lib/api/entities.rb

@@ -205,6 +205,9 @@ module API
       expose :attachment_identifier, as: :attachment
       expose :author, using: Entities::UserBasic
       expose :created_at
+      expose :system?, as: :system
+      expose :upvote?, as: :upvote
+      expose :downvote?, as: :downvote
     end
 
     class MRNote < Grape::Entity