Admin notes in user management

Is your feature request related to a problem? Please describe.
When managing a nextcloud instance with a larger amount of users you might want to add an internal comment to a user. ATM this doesn't seem possible.

Describe the solution you'd like
In the user management view there should be a way to write a note referring to a user and a way to see that there was something noted. It should not be shown to group subadmins, unless explicitly specified through a checkbox.

A more sophisticated solution might include a date picker, where a reminder to the admin group (or maybe even a selectable person in the admin group) on this particular note could be set.

Describe alternatives you've considered
This could also be handled as comments to users, porting the existing comments functionality e.g. from files app. The icon in the below design mockup could then be the comments balloon instead.

Additional context

The issue names no files or tests; begin at the user management view and compare the existing comments functionality referenced in the body, using the linked mockup as context. Before implementation, resolve the open choices around note visibility, comments versus notes, and reminders; done should be a documented, agreed scope.