Zbay should check that every username registration message is signed by our node

[holmesworcester]

This is one step in moving username registrations away from Zcash. (Part of #787.)

In this step, we check that username registration messages are signed by our node, by using a hardcoded public key.

Acceptance criteria: if a message is signed, the username registration is treated as valid, and appears. If it is not, it is invalid, hidden, and cannot be used.

Note: I'm not sure what to do with old accounts here. We could exempt account before a certain block from this check, or we could resend all registration messages but sign them this time. .