Disable User Signup by default

In the vein of Security I think it might be worth considering disabling User Signups by default and making it opt in for users.

This way we reduce the risk by default of users deploying GitLab instances with signup.

We were doing this in GET but since it's an API setting our code would be disabling the signup each run on customer environments so it wasn't appropriate (and we were veering away from Omnibus defaults).