feature(client): Add a `/cookies_disabled` page.

* Add a cookies_disabled template, view, and test. Provides a link to SUMO to get help as well as a "try again" button.
* Split off most of the code from main.js into lib/start-app.js so that it can be tested.
* Split off config loading into its own modules, lib/config-loader.js so that it can be shared.

fixes #738