migrations: add OWNED BY for sequence

The OWNED BY option causes the sequence to be associated with a specific
table column, such that if that column (or its whole table) is dropped,
the sequence will be automatically dropped as well.

Change-Id: Ieedbf0a93e3fd7f7567417f128f291c53f3358d8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/296449
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

migrations/000072_add_documentation_goos_goarch_id.down.sql

@@ -9,6 +9,5 @@ ALTER TABLE documentation DROP COLUMN new_goos;
 ALTER TABLE documentation DROP COLUMN new_goarch;
 DROP TRIGGER documentation_id_update ON documentation;
 DROP FUNCTION update_documentation_id;
-DROP SEQUENCE sequence_documentation_id;
 
 END;

migrations/000072_add_documentation_goos_goarch_id.up.sql

@@ -15,6 +15,7 @@ BEGIN
     RETURN NEW;
 END
 $BODY$ LANGUAGE PLPGSQL;
+ALTER SEQUENCE sequence_documentation_id OWNED BY documentation.id;
 
 CREATE TRIGGER documentation_id_update
 BEFORE INSERT OR UPDATE ON documentation