Prevent non-registered users from creating Nextcloud accounts

[brtptrs]

Describe the bug Everyone can create Nextcloud accounts via signing-requests. On our system, only admins are allowed to create accounts.

To Reproduce Steps to reproduce the behavior:

• Send a request to a mail address of non-users
• External person can register an account, although it is not allowed on our system

Expected behavior

• Only registered users can be invited to sign a document
• No external person should be allowed to register a Nextcloud account via a request to sing a document, sent to an unregistered mail address

[vitormattos]

Related with #686 #1433

[vitormattos]

Would be solve by #1406 Need funding.

[vitormattos]

Fixed at newest release. Now is possible to uncheck the option "request to create account".