Making it possible to disable registration

[kuro-codes]

Is your feature request related to a problem? Please describe.
It would be nice to have a small private instance for a family or a group of friends. So disabling the registration after everybody is on board would achieve this.

Describe the solution you'd like
Create an option in a config file to disable registration and only allow login.

Describe alternatives you've considered
Later on create a Settings page in an admin interface to change it on the fly.

Additional context
Nothing to add here.