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 lead them to configure authentication.

(This could probably use some UI and copy tweaks.)

Test Plan:
{F46738}

{F46739}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6228