Validate primary and secondary metrics overlap in review serializer

[jaredlockhart]

In the old Nimbus UI, we could use react to dynamically remove options from either the primary or secondary outcomes fields when they're selected so that no outcome is marked as both a primary and secondary outcome. In theory we could do something like this with HTMX but it might be a bit hairy, so instead let's just move this into the review validation serializer.

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