Don't allow creating approval rules with approvels required set to 0.
In !15378 (comment 228700410) we're going to allow any developer to approve a merge request, regardless if they are apply to one of the rules on that merge request.
That makes it not necessary to allow users to create approval rules with 0 approvals required. Since those rules don't actually enforce anything.
Should we add validation that ensures that we don't allow users to create those rules?