app: Prepare JS files for consumption by webpack.

This commit prepares the frontend code to be consumed by webpack.

It is a hack: In theory, modules should be declaring and importing the
modules they depend on and the globals they expose directly.

However, that requires significant per-module work, which we don't
really want to block moving our toolchain to webpack on.

So we expose the modules by setting window.varName = varName; as
needed in the js files.

static/js/scroll_util.js

@@ -57,3 +57,4 @@ return exports;
 if (typeof module !== 'undefined') {
     module.exports = scroll_util;
 }
+window.scroll_util = scroll_util;