Better language settings

Language settings are more aligned now with the deployment scenario.

* Moved some static paths out of local config, and just used SITE_ROOT in the local_settings.py file. This defaults to a sensible location if it's missing.
* Changed the working directory of the app to the `python` dir, so that deployment matches production, on Amazon Linux, instead of Ubuntu.
* Added feature to keep application on the same page when the language switcher is switched.
* Added a flag for the current language next to the `Language` dropdown.