Add user id to api trace output

app/views/api/traces/_trace.builder

@@ -3,6 +3,7 @@
 attrs = {
   "id" => trace.id,
   "name" => trace.name,
+  "uid" => trace.user_id,
   "user" => trace.user.display_name,
   "visibility" => trace.visibility,
   "pending" => trace.inserted ? "false" : "true",

test/controllers/api/traces_controller_test.rb

@@ -52,6 +52,7 @@ module Api
       auth_header = basic_authorization_header public_trace_file.user.display_name, "test"
       get api_trace_path(public_trace_file), :headers => auth_header
       assert_response :success
+      assert_select "gpx_file[id='#{public_trace_file.id}'][uid='#{public_trace_file.user.id}']", 1
     end
 
     # Check an anonymous trace can't be specifically fetched by another user