Previously "system_migrations" were silently not applied, because the
migration script tries to find them relatively to its own location, but
here they are not in the same /bin folder that migrate.py is placed in.
* switched from flake-utils to flake-parts
* dropped the overlay and instead populate configurable options for all
required authentik components in the module
* `nixosModule.default` is now a top-level output following the flake spec,
instead of the previously incorrect system-specific definition
