feature #4: create function for follow

resources/functions/follow/unfollow.sql

@@ -0,0 +1,16 @@
+create or replace function unfollow(reference regclass, target_id uuid, citizen_id uuid) returns void
+    language plpgsql as
+$$
+declare
+    _citizen_id alias for citizen_id;
+    _target_id alias for target_id;
+begin
+    delete
+    from follow f
+    where f.citizen_id = _citizen_id
+      and f.target_id = _target_id
+      and f.target_reference = reference;
+end;
+$$;
+
+-- drop function if exists unfollow(regclass, uuid, uuid);