Add UI to mass delete dead bots

appengine/swarming/ui/res/imp/tasklist/task-mass-cancel.html

 <!--
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <task-mass-cancel>
 
   Usage:
 
     <task-mass-cancel></task-mass-cancel>
 
   task-mass-cancel offers an interface for the user to cancel multiple tasks
   (and hopefully avoid doing so on accident).
 
   Properties:
     //input
-    auth_headers:
+    auth_headers: Object, the OAuth2 header to include in the request.  This
+        should come from swarming-app.
     tags: Array<String> the tags that match the tasks being canceled.
 
     // output
-    started_canceling: Boolean, If the user has hit the big red button to start canceling tasks.
-    finished_canceling: Boolean, If the dialog has finished canceling all the tasks.
+    started_canceling: Boolean, If the user has hit the big red button to start
+      canceling tasks.
+    finished_canceling: Boolean, If the dialog has finished canceling all the
+      tasks.
 
   Methods:
-    prompt() Set up the dialog, resetting started_canceling and finished_canceling to false.
+    prompt() Set up the dialog, resetting started_canceling and
+      finished_canceling to false.
 
   Events:
     None.
 -->
 <link rel="import" href="/res/imp/bower_components/paper-button/paper-button.html">
 
 <dom-module id="task-mass-cancel">
   <template>
     <style>
       .container {
@@ skipping 29 matching lines @@
       <paper-button
           class="cancel-button"
           raised
           disabled$="[[!_ready_to_cancel]]"
           hidden$="[[started_canceling]]"
           on-tap="_cancel">
           Cancel the Tasks
       </paper-button>
 
       <div >
-        <div hidden$="[[!started_canceling]]">Progress: [[_canceled_count]] queued for cancelation</div>
+        <div hidden$="[[!started_canceling]]">Progress: [[_canceled_count]] queued for cancellation</div>
         <div>Note: tasks queued for cancellation will be canceled as soon as possible, but there may be some delay between when this dialog box is closed and all tasks actually being canceled.</div>
       </div>
 
     </div>
 
   </template>
   <script>
   (function(){
     var CANCEL_BATCH_SIZE = 100;
     Polymer({
-      is: 'task-mass-cancel',
+      is: "task-mass-cancel",
 
       properties: {
         // input
         auth_headers: {
           type: Object,
         },
         tags: {
           type: Array,
         },
         // output
@@ skipping 79 matching lines @@
           }.bind(this))
           .catch(function(e){
             console.log(e);
             sk.errorMessage(e);
           });
       },
     });
   })()
   </script>
 </dom-module>