Add a registration/set-up page for new admins

[presleyp]

Problem

Admins of new deployments can only set up their first user through the CLI

Solution

In a deployment with no users, the login page redirects to a setup page that has a UI for the setup process

Definition of Done

• [ ] In a deployment with no users, the login page redirects to a setup page with a form
• [ ] The form takes a username, email, and password
• [ ] Submitting the form creates a user and logs them in

Notes

Here's what the signup flow looks like from the CLI: https://asciinema.org/a/cy8QVTPwazDAV6cxcV6DF1Qnj Here's where initial user creation is implemented in the CLI: https://github.com/coder/coder/blob/v0.8.1/cli/login.go#L81-L196