Add unique index for to ApprovalMergeRequestRules for "rule_type=regular"

While reviewing my changes to approval rule indices, @patrickbajao commented (!33121 (comment 356677173))

I just realized we don't have a unique index for non-code owner rules (rule_type as regular). Not sure why we don't have one (we have a Rails validation for it). I think we should add one.

We also need to ensure that we migrate existing records as there's a ~bug that allows adding approval rules with the same name: #247119 (closed).