fix(client): Go to the /cookies_disabled screen instead of the /500 screen if cookies are disabled.

The User model was accessing localStorage directly which causes most browsers to except. If localStorage is disabled, create a Storage object with a NullStorage backing instead.

fixes #1919
This would be a decent option. [Here](https://github.com/mozilla/fxa-content-server/issues/1921) I also suggest using a factory method on `Storage` to get a safely instantiated storage object.