Tunable Justification Generator (#835)

* Add way to create tunable GRANDPA justifications

* Use new function in tests

* Allow multiple authorities on a single fork

* Only store pre-commit targets instead of full ancestry chains

* Rename precommit_header to be more generic

* Push new digest item instead of overriding entire digest

* Ensure that we generate chains with non-zero length

* Extract justification creation parameters into struct

* Appease Clippy