Add bot-page summary with utilization stats

appengine/swarming/elements/res/imp/botpage/bot-page-data.html

 <!--
   Copyright 2016 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.
 
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <bot-page-data>
 
@@ skipping 201 matching lines @@
               task[time] = new Date(task[time]);
               task["human_"+time] = formatDate(task[time]);
             }
           });
 
           if (task.duration) {
             task.human_duration = this._humanDuration(task.duration);
           } else {
             var end = task.completed_ts || task.abandoned_ts || task.modified_ts || new Date();
             task.human_duration = this._timeDiffExact(task.started_ts, end);
+            task.duration = (end.getTime() - task.started_ts) / 1000;
           }
 
           task.state = task.state || "UNKNOWN";
           if (task.state === "COMPLETED") {
             if (task.failure) {
               task.state = "FAILURE";
             } else {
               task.state = "SUCCESS";
             }
           }
 
         }.bind(this));
 
         // Sort the most recent tasks first.
         tasks.sort(function(a,b) {
           return b.started_ts - a.started_ts;
         });
 
         return tasks;
       }
 
     });
   })();
   </script>
 </dom-module>