BABE's revert procedure (#11022)

* First rough draft for BABE revert

* Proper babe revert test

* Cleanup

* Test trivial cleanup

* Fix to make clippy happy

* Check polkadot companion

* Check cumulus companion

* Remove babe's blocks weight on revert

* Handle "empty" blockchain edge case

* Run companions

* Simplify the filter predicate

* Saturating sub is not required

* Run pipeline

* Run pipeline again...