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
Merge pull request #1920 from mozilla/issue-1919-cookies-disabled-explosion

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