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/server_events.js

@@ -296,3 +296,4 @@ return exports;
 if (typeof module !== 'undefined') {
     module.exports = server_events;
 }
+window.server_events = server_events;