* Explicitly require the polyfills bundle from the sidebar
   and injector. In future we'll want to avoid including these
   polyfills for newer browsers but requiring it explicitly
   makes it obvious if the bundle is missing in the meantime.

 * Simplify the configuration specifying the modules to
   skip parsing for, since there are only two.

 * Reduce duplicated logic for creating app bundle configuration

 * Add additional comments and documentation to explain
   what various parts of the config do

 * Rename createBundle.js -> create-bundle.js for consistency with
   the rest of the codebase and move vendor-bundles.js to
   scripts/gulp to avoid cluttering the top of the source tree.