SSNList contract can accumulate funds without attributing them to any entity

[anton-trunov]

The AddFunds transition accepts funds from Proxy which does not have any
further checks, which means anyone can send money to SSNList (via Proxy) and the
contract will accept it. What if this happens by mistake? Should the contract
unconditionally accept funds?

[renlulu]

I remember we are meaning to have this design? Not too sure.

[renlulu]

@bb111189