Update name validation and index on ApprovalMergeRequestRule

(Broken out from #12137 (comment 327671181))

We’ll need to update the unique index, which is currently:

CREATE UNIQUE INDEX approval_rule_name_index_for_code_owners ON public.approval_merge_request_rules USING btree (merge_request_id, code_owner, name) WHERE (code_owner = true);

(Add in db/migrate/20190402150158_backport_enterprise_schema.rb)

We also want to update

index_approval_rule_name_for_code_owners_rule_type

---

We’ll additionally need to update the name validation/unique index to add the scope of :section

# ee/app/models/approval_merge_request_rule.rb:29
#
validates :name, uniqueness: { scope: [:merge_request_id, :rule_type] }`

Supports #12137 (closed)