✨(organizations) add siret to name conversion

This adds the plugin system to easily manage
Organization related customizations. This first
plugin tries (best effort) to get a proper name
for the Organization, using its SIRET. This
is French specificities but another plugin can
be defined for other cases.

src/backend/people/settings.py

@@ -473,6 +473,11 @@ class Base(Configuration):
             environ_prefix=None,
         )
     )
+    ORGANIZATION_PLUGINS = values.ListValue(
+        default=[],
+        environ_name="ORGANIZATION_PLUGINS",
+        environ_prefix=None,
+    )
 
     # pylint: disable=invalid-name
     @property