Disable registration

[Tazmanian79]

You should be able to disable the registration.

Now if for example a bot finds the login screen 1000 accounts kan be registered without knowing.
Also an account should have admin options to see which accounts exist.

[Freika]

Free registration was disabled in https://github.com/Freika/dawarich/releases/tag/0.8.1. Accounts management and basic administration functions will be introduced later on