Add a file backend

The new backend uses the filesystem as backing storage. The backing directory is migratable thanks to support for relative symlinks that will never break as long as the directory state is consistent.

Usage: launch Kittybox with BACKEND_URI=file://$folder where $folder is an absolute (or a relative path - might also work) path to the folder (the folder must exist, or the user account running Kittybox must have permissions to create the folder and all its missing components).