It appears to be an omission that ALTER TABLE ... ENABLE TRIGGER and
similar commands don't allow acting on views, even though we now have
triggers on views.
Similarly, the ALTER TABLE ... ENABLE RULE commands only allow acting on
tables, even though rules can also exist on views and materialized views.
(Why don't we allow rules on foreign tables? Is that intentional?)
Attached is a sample patch. It appears we don't have any regression
tests for this.
