WARNING: Invited admins might not be validated

Users can become non-validated admins by being invited in an item as admin.

Currently it is not a problem, because users always sign up using their emails.
However adding the possibility to signup with a password could lead to non-validated admins.

This is problematic when packed items, memberships returns admin but the backend does not allow operations from non-validated members.

A case that happened to us is an application trying to post app settings as non-validated admin. This case was mostly problematic because the operation was looping.