Backend Validation on register

[clxxiii]

Currently there is a bug with I forgot to fix with the signup button, where it still shows up even if signups are closed.
The issue is that while registrations were closed, I was still able to sign up, but the settings page disappeared so I couldn't unregister

It needs to be validated on the:

• Frontend
• Backend

To make sure people can't sign up when registrations are closed.