fix: migrate management commands from apis-webpage to apis-rdf-core

This commit introduces a manangement command that was part of the
`webpage` module but is better suited to be part of `apis_core`.

apis_core/management/commands/create_relationships.py

@@ -0,0 +1,41 @@
+from django.core.management.base import BaseCommand, CommandError
+from apis_core.apis_relations.models import Property
+from apis_ontology.models import construct_properties
+
+
+class Command(BaseCommand):
+    help = (
+        "Create and label relationships between entities by running "
+        "the construct_properties() function in your app's models.py file."
+    )
+
+    def handle(self, *args, **options):
+        try:
+            construct_properties()
+            self.stdout.write(
+                self.style.SUCCESS("Successfully ran construct_properties()")
+            )
+        except Exception as e:
+            raise CommandError(
+                f"construct_properties() raised the " f"following error(s):\n{e}"
+            )
+
+        try:
+            props = Property.objects.all()
+        except Property.DoesNotExist:
+            raise CommandError("No entity relationships exist.")
+
+        if len(props) > 0:
+            # TODO would ideally verify existence/validity of relevant fields
+            #  as relationship names alone are useless w/o subj/obj references;
+            #  below msg. purposefully only mentions "labels".
+            self.stdout.write("The following labels now exist:")
+            for p in props:
+                self.stdout.write(self.style.SUCCESS(f"{p.name}"), ending="")
+                self.stdout.write(f" ... {p.name_reverse}")
+        else:
+            # construct_properties() starts out by deleting all existing
+            # Property objects
+            self.stdout.write(
+                self.style.ERROR("No entity relationships found nor created.")
+            )