A Social Account Recovery Pallet (#4531)

* Initial sketch of social recovery pallet

* Fix compilation issues

* Use a single total delay, rename stuff

* Check possible overflow

* Copyright bump

* Add mock for tests

* Add basic end to end test

* Add `create_recovery` tests

* Add malicious recovery lifecycle test

* Make clear we check for sorted and unique friends

* Work on some tests, clean up imports

* Change `if let Some(_)` to `ok_or()`

* More tests

* Finish tests, except issue with `on_free_balance_zero`

* Fix `on_free_balance_zero`

* Pallet docs

* Add function/weight docs

* Fix merge master

* OnReapAccount for System too

* Update weight docs

* Allow passthrough to support fee-less extrinsics