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
Perhaps we should remove references to `localStorage` from this file and always passed in the `storage` option that has been properly instantiated from `app-start.js`. The default could just use `NullStorage`.