Add admin-controlled toggle to disable new user signups

[bpepple]

Summary

• Adds a SignupSettings singleton model (editable from Django admin) with a signups_enabled flag and a customizable disabled_message
• When disabled, /accounts/signup/ renders a friendly "Signups Disabled" page instead of the registration form, for both GET and POST
• Lets staff close/reopen signups instantly from the admin, with no code deploy or restart — replaces the old approach of commenting out the URL route and hand-editing the navbar template

Motivation

We've been seeing a steady stream of duplicate account creation. Signups have been disabled twice before in this repo by editing code and reverting later, which is easy to forget to undo. This gives us a proper on/off switch.