Need a new role for user management only in private Gallery

[1737280]

Currently there's only the Curator role that can perform admin functions in Gallery. This role is too broad. We would like to have a separate admin role that can only perform user management but nothing else. Likewise, we would like an admin role that can manage workflows/collections, view metrics etc, but cannot do user management (view/add/edit/delete users). Such more fine-grained roles are necessary in IT management where strict separation of duties is a mandate.