Integrate Approval Voting into Overseer / Service / GRANDPA (#2412)

* integrate approval voting into overseer

* expose public API and make keystore arc

* integrate overseer in service

* guide: `ApprovedAncestor` returns block number

* return block number along with hash from ApprovedAncestor

* introduce a voting rule for reporting on approval checking

* integrate the delay voting rule

* Rococo configuration

* fix compilation and add slack

* fix web-wasm build

* tweak parameterization

* migrate voting rules to asycn

* remove hack comment