Refactor prior to adding task-page

appengine/swarming/elements/res/imp/common/swarming-app.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:
 
     <swarming-app>
 
@@ skipping 215 matching lines @@
         min-width: 5.14em;
         background-color: #fff;
         color: #1f78b4;
         text-align: center;
         text-transform: uppercase;
         outline: none;
         border-radius: 3px;
         padding: 0.6em 1.2em;
         border: solid lightgray 1px;
         margin: 0.6em;
+        cursor: pointer;
       }
 
       button:hover {
         background: #eee;
       }
 
       button:focus {
         background-color: #ddd;
         transition: background-color 0.1s cubic-bezier(0.4, 0, 0.2, 1);
       }
@@ skipping 41 matching lines @@
         border: none;
         line-height: 20px;
         vertical-align: middle;
       }
 
       select {
         overflow-y: auto;
       }
   </style>
 </dom-module>