PolyGerrit: Rework script imports

With Buck, we won't easily be able to follow the <!-- build:js -->
directive in index.html to combine multiple .js files into a single
app.js. Instead, move those includes into the top-level component
gr-app.html, so vulcanize will simply inline them. This leaves
index.html as a simple unvulcanized shim that will just load the
polyfills and the top-level app.

After this change, we can vulcanize gr-app.html and just swap out one
path in index.html, otherwise serving it as-is.

Change-Id: I782c4bbd259c03c2d7e2d33342462510a91f32e3

polygerrit-ui/app/index.html

@@ -44,11 +44,3 @@ window.Polymer.dom = 'shadow';
 <template is="dom-bind" id="app">
   <gr-app params="{{params}}" route="{{route}}"></gr-app>
 </template>
-
-<!-- build:js /scripts/app.js -->
-<script src="/bower_components/page/page.js"></script>
-<script src="/scripts/app.js"></script>
-<script src="/scripts/changes.js"></script>
-<script src="/scripts/util.js"></script>
-
-<!-- endbuild-->