`approval-voting`: merge tranche 0 assignments

[sandreim]

As pointed out in https://github.com/paritytech/polkadot/issues/6610#issuecomment-1401468099 we can save a lot of CPU and reduce gossip by doing only one RelayVrfModulo signature per authority and derive multiple assignments from the output which stacks with batch verification.

What it is not clear at least for me is if we can also merge Delay assignments per tranche for the same signer by signing once for multiple candidates. Depending on the distribution this can also save us CPU when importing and reduce the network load.

Another question is why would we need authorities to refuse some assignments ? Is this for being able to control the amount of work to be done ?

CC @burdges @eskimor