manager for sport_obj, event_grp_obj and event_obj

[Dimfred]

Introduces a manager attribute for the objects above.
Every manager, higher in the object hierachy can also modify objects below (sport can mod eventgrp & event).

[xeroc]

Excellent work on the pull request ... need community/management to agree on it for merging.

@Dimfred can you provide a specification so we have a PIP?

[pmconrad]

All these changes are modifying consensus rules and need hardfork protection.

[xeroc]

Interestingly, We have Rules objects that do not have a parent (and thus cannot be managed). As a consequence, witness-account needs to approve every new Rule on the chain!