Provide API for user's identity check

[vporton]

Motivation

We need a way to check if two different accounts are associated with the same human or not. It is called user identity or anti-Sybil.

This task became especially important because of MPlus4Climate/MPlusToolKit#3 (comment)

Details

For anti-Sybil check if user’s private key is associated with a GoodDollar UBI account. To persuade somebody to give away his private key, one probably needs to pay him more than his basic income. So to get many fake accounts costs rather much, moreover, if we receive somebody’s private key, he can be in control of his funds (all tokens and FUSE associated with that account, not only his basic income).

Feature request: Provide an official API to easily do it using GoodDollar.

See also: aragon/client#1524

[vporton]

Also need API to check if an address has claimed recently (or even better determine the time of his or her last claim).