Enable chained root delegations

[danfinlay]

Per delegatable/delegatable-sol#30

Since:

• we want users to opt into their own "roots of trust"
• we want users to be able to report phishers once, and have those reports respected regardless of who trusts them

Therefore:

• We may want to use a variant of Delegatable that allows "chained root delegations" per Should root delegations work in a chain? delegatable-eth#9

With this change, anyone could deploy their own copy of the registry (maybe counterfactually!), but still with just a couple delegations chain together with outstanding phisher-reporter delegations and reports, making the trust infrastructure much more dynamic and decentralized.