fix the modified_by and modified_at columns

As all changes are already written into history, why not enforce the
proper changing of modified_at and modified_by columns?

This way we can be sure these are properly changed and don't have to
take care of that in the SQL statements anymore.

schema/01_initial.sql

@@ -155,6 +155,13 @@ comment on column history.action is 'The action can be one of insert, update or
 
 create or replace function record() returns trigger as $$
 begin
+	if TG_OP = 'UPDATE' or TG_OP = 'INSERT' then
+		NEW.modified_at = now();
+		NEW.modified_by = current_setting('dim.username');
+	end if;
+	if TG_OP = 'INSERT' then
+		NEW.created_by = current_setting('dim.username');
+	end if;
   insert into history(transaction, created_by, type, action, old_entity, new_entity)
   values (
     current_setting('dim.transaction'),