Remove gr-app.html and all its reference

Use gr-app.js as the entry and shell.
https://polymer-library.polymer-project.org/3.0/docs/tools/polymer-json

Change-Id: Iacd7a433157355f89d7f8e6c782abfe43ba725ea

resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy

@@ -139,7 +139,7 @@
   // Content between webcomponents-lite and the load of the main app element
   // run before polymer-resin is installed so may have security consequences.
   // Contact your local security engineer if you have any questions, and
-  // CC them on any changes that load content before gr-app.html.
+  // CC them on any changes that load content before gr-app.js.
   //
   // github.com/Polymer/polymer-resin/blob/master/getting-started.md#integrating
   {if $assetsPath and $assetsBundle}