Restrict sharing to admins or single users only

[wdfee]

1. Using auto groups enables all users to send files, mails and so on to all other users with a single click. Especially with a larger user base this possibility should be restricted to admin and/or authorized users.
2. The sharing option "only autocomplete users in the same groups as you" doesn't work with auto groups anymore, because everybody is in the same group.

This issue is equivalent to the group_everyone's issue "add option to restrict usage to admins" icewind1991/group_everyone#11
and the mail's app issue "[Feature Request] Restrict sending emails to specific groups" nextcloud/mail#1862

[stjosh]

Thanks for your suggestions! I'll have to look into how to achieve this, i.e., "hide" the AutoGroups for certain users and use-cases.

Maybe I can make use of the the AutoComplete event introduced in nextcloud/server#14385 - will need to find out whether hiding it from AutoComplete is already enough.