Add pageable data widget

appengine/swarming/elements/res/imp/common/common-behavior.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.
 
   It contains the definition of the following Behaviors:
 
     SwarmingBehaviors.CommonBehavior
 
   To use it, include
@@ skipping 126 matching lines @@
       // now is used.
       _timeDiffExact: function(first, second){
         if (!first) {
           return "eons";
         }
         if (!second) {
           second = new Date();
         }
         return this._humanDuration((second.getTime() - first.getTime())/1000);
       },
+
+      _truthy: function(a){
+        return !!a;
+      }
     };
   })();
 </script>