Rococo: ability to programatically assign slots to teams (#3943)

* Permanent & Temp parachain slots on Rococo - WIP

* Revert test change

* Revert test change

* Fix formatting

* Extract logic to separate assigned_slots pallet

* Formatting

* Parachain downgrade logic

* Pallet doc comment

* Revert unnecessary changes

* Fix few issues, tweak temp slots allocation logic; add a bunch of tests

* Address review comments; track active temp slots

* Update runtime/common/src/assigned_slots.rs

* Update runtime/common/src/assigned_slots.rs

* Remove assigned_slots calls from paras_sudo_wrapper

* Unassign is a perfectly valid verb

* Remove unneeded collect

* Update code following #3980

* Cleanup

* Generate storage info for pallet

* Address review comments

* Add ForceOrigin to slots pallet

* Track permanent slot duration in storage

* Fix tests build

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>