fix(i18n): Ensure i18n works in Chrome.

* Remove the bulk of the client side code used to determine the user's language.
* Use the `Accept-Language` header to allow the server to determine which language to show to the user.

fixes #823
Merge pull request #916 from mozilla/issue-823-l18n-on-server

fix(i18n): Ensure i18n works in Chrome.