Make approval required the default

When launching a new site on open internet, it's not particularly safe to have open registrations by default (even with captcha enabled). We don't know what the admin's purpose or plan is, and they might have to rush to close registrations.

Requiring approval by default makes the most sense. It's likely what the majority of admins want, and there's no harm in it if users try to sign up before an admin changes the setting.