Assign limited # of invite codes to existing users

[nicnab]

In cases where people use nextcloud to host instances for a certain community or interest group, it might be interesting to be able to give existing users a limited number of invite codes they can give to their friends. Then there would have to be a mode the admin can put his instance into where you can only register a new account using one of those existing codes.

Not sure if anyone here remembers but 15 years ago those invites were one of the main reasons why gmail was so well received among tech people.

[pellaeon]

This is an enhancement, I will not work on it but pull requests welcome.

[nickvergessen]

Also related #158