add invite hint to console login page

[qrkourier]

add a zrok controller config option for a signup page url to be displayed on the console login page or add some default usage hints to the login page like "Install zrok CLI from here and run zrok invite to get started".

why: reduce zrok.io signup friction

hypothetical: if a prospective zrok.io signup visits zrok.io and sees the "Log in" button, they may click that before learning how to invite themselves with the CLI. It's reasonable for that prospect to expect a "sign up" or "get an invitation" link or a hint about inviting themself with the CLI, or both.