Introduce `cancel_proposal` to rid us of those pesky proposals (#7111)

* Introduce `cancel_proposal`

Also fix proposal weight.

* Support proposal cancellation from runtime.

* Fixes

* Fixes

* Fixes

* Fixes

* Fixes

* Fixes

* Fixes

* Fix benchmarks

* fix benchmark

* whitelisted caller weights

* fix build

* Fixes

* Fixes

* Fixes

* Fixes

* Update frame/democracy/src/lib.rs

Co-authored-by: Shawn Tabrizi <[email protected]>

* Fixes

* Fixes

* Fixes

* Fixes

* Fixes

* doc updates

* new weights

Co-authored-by: Shawn Tabrizi <[email protected]>