Make OAuth client id accessible to new ui w/o hardcoding

appengine/swarming/elements/res/imp/botlist/bot-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:
 
     <bot-list>
 
   bot-list creats a dynamic table for viewing swarming bots.  Columns can be
   dynamically filtered and it supports client-side filtering.
 
+  This is a top-level element.
+
   Properties:
-    None.  This is a top-level element.
+    client_id: String, will be set by server-side template evaluation.
 
   Methods:
     None.
 
   Events:
     None.
 -->
 
 <link rel="import" href="/res/imp/bower_components/iron-flex-layout/iron-flex-layout-classes.html">
 <link rel="import" href="/res/imp/bower_components/polymer/polymer.html">
@@ skipping 43 matching lines @@
         right: 0;
         top: 0.4em;
       }
       .bot-list th > span {
         /* Leave space for sort-toggle*/
         padding-right: 30px;
       }
     </style>
 
     <swarming-app
+      client_id="[[client_id]]"
       auth_headers="{{_auth_headers}}"
       signed_in="{{_signed_in}}"
 
       busy="[[_busy]]"
       name="Swarming Bot List">
 
       <h2 hidden$="[[_signed_in]]">You must sign in to see anything useful.</h2>
 
       <div hidden$="[[_not(_signed_in)]]">
 
@@ skipping 287 matching lines @@
         return dir * swarming.naturalCompare(botACol, botBCol);
       },
     };
 
     Polymer({
       is: 'bot-list',
       behaviors: [SwarmingBehaviors.BotListBehavior],
 
       properties: {
 
+        client_id: {
+          type: String,
+        },
+
         _bots: {
           type: Array,
         },
 
         _columns: {
           type: Array,
         },
 
         _filter: {
           type: Function,
@@ skipping 137 matching lines @@
         if (data && data.task_id) {
           return "/user/task/" + data.task_id;
         }
         return undefined;
       }
 
     });
   })();
   </script>
 </dom-module>