Add support for the XDG Base Directory Specification

Please consider supporting freedesktop.org's specifications about config directory location.

References: http://freedesktop.org/wiki/Software/xdg-user-dirs/, http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables.

See additional discussion: https://github.com/mullvad/mullvad-browser/issues/224.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information