Milo: Add task expired as a failure status

milo/appengine/frontend/static/buildbot/css/default.css

 /* Copyright 2015 The LUCI Authors. All rights reserved.
  * Use of this source code is governed under the Apache License, Version 2.0
  * that can be found in the LICENSE file.
  */
 
 body.interface {
   margin-left: 30px;
   margin-right: 30px;
   margin-top: 20px;
   margin-bottom: 50px;
@@ skipping 435 matching lines @@
   background-color: #fa3;
   border-color: #C29D46;
 }
 
 .status-InfraFailure {
   color: #FFFFFF;
   background-color: #c6c;
   border-color: #ACA0B3;
 }
 
+.status-Expired, .expired {
+  color: #FFFFFF;
+  background-color: #ac39ac;
+  border-color: #ACA0B3;
+}
+
 .warnings {
   color: #FFFFFF;
   background-color: #fa3;
   border-color: #C29D46;
 }
 
 .skipped {
   color: #000;
   background: #AADDEE;
   border-color: #AADDEE;
@@ skipping 245 matching lines @@
   display: inline;
 }
 
 div.step-text:first-child {
     display: inline;
 }
 
 .verbosity-Hidden.default {
   display: none;
 }