R1:32f6c88896c8

Add first-time-setup registration flow

Summary:
Ref T1536. Currently, when you install Phabricator you're dumped on the login screen and have to consult the documentation to learn about `bin/accountadmin`.

Instead, detect that an install is running first-time setup:

- It has no configured providers; and
- it has no user accounts.

We can safely deduce that such an install isn't configured yet, and let the user create an admin account from the web UI.

After they login, we raise a setup issue and…