Revert chain if at least f+1 validators voted against a candidate (#7151)

* Make `issue_explicit_statement_with_index` regular function

* Make `issue_backing_statement_with_index` regular function

* Issue `RevertBlocks` as soon as a dispute has `byzantine threshold + 1` invalid votes.

* Remove a comment

* Fix `has_fresh_byzantine_threshold_against()`

* Extend `informs_chain_selection_when_dispute_concluded_against` test