Refactor post requests, implement bot cancel/terminate

appengine/swarming/elements/res/imp/tasklist/task-list.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:
 
     <task-list>
 
@@ skipping 314 matching lines @@
       },
 
       _cancelTask: function(e) {
         var task = e.model.task;
         if (!task || !task.task_id) {
           console.log("Missing task info", task);
           return
         }
         var id = task.task_id
 
-        // Keep toast displayed until we hear back from the cancel.
-        this.$.toast.duration = 0;
         this.$.toast.text="Canceling task " + id;
-        this.$.toast.open();
         var url = "/_ah/api/swarming/v1/task/" + id +"/cancel";
-        sk.request("POST", url, undefined, this._auth_headers).then(function(response) {
-          this.$.toast.close();
-          this.$.toast.show({
-            text: "Request sent.  Response: "+response,
-            duration: 3000,
-          });
-        }.bind(this)).catch(function(reason) {
-          console.log("Cancellation failed", reason);
-          this.$.toast.close();
-          this.$.toast.show({
-            text: "Request failed.  Reason: "+reason,
-            duration: 3000,
-          });
-        }.bind(this));
+        this._postWithToast(url, this.$.toast, this._auth_headers);
       },
 
       _tag: function(task, col) {
         if (!task || !task.tagMap) {
           return undefined;
         }
         return task.tagMap[col];
       },
 
       _taskLink: function(taskId) {
@@ skipping 18 matching lines @@
         if (state === "RUNNING" || state === "PENDING") {
           return "pending";
         }
         return "";
       }
 
     });
   })();
   </script>
 </dom-module>