User profile and Administration icons location should be customizable

Currently the user profile and administration icons location are not customizable. They are not defined in slots.

See here:
https://github.com/nuxeo/nuxeo-web-ui/blob/9.10/elements/nuxeo-app.html#L231

and here:
https://github.com/nuxeo/nuxeo-web-ui/blob/9.10/elements/nuxeo-app.html#L234

Having them in slots would allow customers to entirely have their UI tailored as they want it.