An always-on Zbay (not our cloud node) should handle username registrations

[holmesworcester]

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

In this step, we make it so that Zbay itself, with a certain admin account, and not our cloud node, handles username registrations and signs all keys.

(In subsequent steps the owner of a community will be the one to receive and sign username registration requests.)