Allow loading settings from outside of the postgresqleu module

Allow loading settings from outside of the postgresqleu module

To ease building docker images and prevent having to maintain an
in-tree file for local settings, allow a pgeu_system_global_settings
module anywhere in the PYTHONPATH for configuration.

In addition, also allow overrides to be applied after loading the
skin through a pgeu_system_override_settings module in PYTHONPATH.