Allow loading settings from outside of the postgresqleu tree

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

postgresqleu/settings.py

@@ -307,7 +307,13 @@
 try:
     from .local_settings import *
 except ImportError as e:
-    pass
+    # If there's no local_settings.py within the postgresqleu tree, check
+    # for a globally available pgeu_system_settings module in any configured
+    # PYTHONPATH.
+    try:
+        from pgeu_system_settings import *
+    except ImportError as e:
+        pass
 
 PRELOAD_URLS = []
 if 'SYSTEM_SKIN_DIRECTORY' in globals():