staking: Proportional ledger slashing (#10982)

* staking: Proportional ledger slashing

* Some comment cleanup

* Update frame/staking/src/pallet/mod.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Fix benchmarks

* FMT

* Try fill in all staking configs

* round of feedback and imp from kian

* demonstrate per_thing usage

* Update some tests

* FMT

* Test that era offset works correctly

* Update mocks

* Remove unnescary docs

* Remove unlock_era

* Update frame/staking/src/lib.rs

* Adjust tests to account for only remove when < ED

* Remove stale TODOs

* Remove dupe test

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: kianenigma <kian@parity.io>