* Move `app/assets/manifest.js` to `app/assets/config/manifest.js`.
  Avoid the suggestion that you can/should deep-link `stylesheets/foo`.
* Pull in all toplevel stylesheets and JavaScripts, not just
  `application.js` and `.css`. Demonstrate how to use `link_directory`
  with a specified `.js`/`.css` type.
* Fix RAILS_ENV handling in assets tests.
* Shush warnings spam from third-party libs that distract from tests.