User Management Enhancement

Issues

Currently using just one node reference may be very restrictive if the user is used across multiple projects.
It should be possible to assign permissions for each user profile (e.g.: a user can have multiple profiles/node references).
The appearance of custom user profiles should be configurable. (e.g: via a schema property)
We need to decide how these profile nodes should be locatable within the UI.
A user should be able to have multiple profiles/nodes This could be solved by changing the node reference to a node list.
It should be possible to upload an avatar image.
We should support gravatar by default.
It should be possible to import users via CSV
The user should be able to update himself without any additional role checks
Schemas must be identifable as user schema in order to be directly listable within the user profile view.
We should invoke load tests with lots of users.