JSON of user stats of the user

    GET parameters:
      html (bool):
        turn on the HTML version of the API, defaults to false
      user (int):
        identifier of the user, defaults to logged user
      concepts (list):
        list of identifiers of concepts, defaults to all concepts
      lang (str):
        language of requested concepts, defaults to language from django

def user_stats(request):
    """
    JSON of user stats of the user

    GET parameters:
      html (bool):
        turn on the HTML version of the API, defaults to false
      user (int):
        identifier of the user, defaults to logged user
      concepts (list):
        list of identifiers of concepts, defaults to all concepts
      lang (str):
        language of requested concepts, defaults to language from django
    """
    user = get_user_id(request)
    language = get_language(request)

    concepts = None    # meaning all concept
    if "concepts" in request.GET:
        concepts = Concept.objects.filter(lang=language, active=True,
                                          identifier__in=load_query_json(request.GET, "concepts"))
    data = UserStat.objects.get_user_stats(user, language, concepts)
    return render_json(request, data, template='concepts_json.html', help_text=user_stats.__doc__)