Onion service should have a random prefix

Until next gen onion services are live, the actual onion addresses aren't private from HSDir nodes. So it's possible that Haven onion services could get discovered by an attacker, giving them access to all of the evidence logs.

The easiest way to thwart this is to generate a random string and prefix all the URLs with it. So instead of starting with just http://blahblahblah.onion:8080/, the URLs should start with http://blahblahblah.onion:8080/randomstring/. This way, if an attacker discovered the onion service, they won't be able to view the logs without guessing the value of randomstring -- which is essentially a random password. This is how OnionShare URLs works.