Refact proposal pallet (#2322)

* Refact proposal pallet

* Update runtime/rococo/src/propose_parachain.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Fix

* Apply suggestions from code review

Co-authored-by: Andronik Ordian <write@reusable.software>

* AHH

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Andronik Ordian <write@reusable.software>
Co-authored-by: Bastian Köcher <info@kchr.de>