Clean up rounds handling in Results Validators

[gregorbg]

There were plenty of small but important (in terms of complexity) places throughout Results Validator logic that had a pretty complicated idea about rounds, getting round data, comparing round IDs, etc.

This PR aims to fully clean these spaces up, now that we have stable, mandatory round ID references on results as well as inbox_results. In practice, this means two things:

• Validations along the lines of "unexpected results for a round which doesn't exist" don't make sense anymore, because every uploaded result has to reference its round.
• Playing switcheroo with Round Type IDs (which are heavily based on cutoff) is not necessary anymore, because you cannot have the "oh you said this was planned as cutoff but actually everyone made cutoff so it got synced as the 'wrong' round type" case anymore. Again, every uploaded result has to clearly reference its round that must be fully in-sync with the "Edit Events" panel.