Update dispute participation on active leaves update

[eskimor]

As described in the guide, it is perfectly possible for a node to learn about a dispute via dispute-distribution before seeing the candidate included on any chain. The node should however bump participation from best-effort to priority as soon as it learns about the inclusion.

We should also update spam slots accordingly - if we were not aware of the chain backing/including the candidate we might have bumped spam slots for the signing validator, that would also need to be reverted.

[eskimor]

With the change of the best-effort queue to be about backed candidates, something similar applies. We might not have seen (because severely out of sync) a block backing a candidate, while already receiving dispute votes. We would not participate in that case, but we should do so after the fact once we learn about the backing block.

[BradleyOlson64]

Leaning towards not increasing spam slots for backed candidates.