Protect instances with a key

[Errorname]

Is your feature request related to a problem? Please describe.
I want to protect my instance with a key so that only the people with the key will be able to create their account.

Describe the solution you'd like
As an admin, I want to generate a key on the settings pages.
As a user, after signing in for the first time, I will be asked a key to validate my registration. After that, every sign in will be as usual.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Protecting the instance with a key must be optional. Some instances need to be open to everyone (e.g: demo.faq.team) and some are already protected with a whitelist of authorized email domains.