Make current user available in calback_context

[robert-norberg]

Adds a hook so that the currently logged in user object can be accessed in any callback by e.g.

@app.callback(Output(...), Input(...))
def some_callback_function(input1):
    ...
    current_user = dash.callback_context.custom_data.get("user")
    if "admin" in current_user.get("groups", []):
        ...
    else:
        ...