Add stub for new polymer-based Isolate UI

appengine/isolate/ui/res/imp/index/isolate-index.html

+<!--
+  Copyright 2017 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:
+
+    <isolate-index>
+
+    Isolate Index is the landing page for the Isolate UI.
+    It will have links to all other pages and a high-level overview of the fleet.
+
+  Usage:
+
+    <isolate-index></isolate-index>
+
+  This is a top-level element.
+
+  Properties:
+    client_id: String, Oauth 2.0 client id.  It will be set by server-side
+        template evaluation.
+
+  Methods:
+    None.
+
+  Events:
+    None.
+-->
+
+<dom-module id="isolate-index">
+  <template>
+    <style>
+      h1 {
+        color:purple;
+      }
+    </style>
+
+    <h1>Hello World</h1>
+
+    <div>{{client_id}}</div>
+
+  </template>
+  <script>
+    Polymer({
+      is: 'isolate-index',
+
+      properties: {
+        client_id: {
+          type: String,
+        }
+      },
+
+    });
+  </script>
+</dom-module>