add new template filter "has_children"

example:

    {% for item in items %}
    <li>
        {{ item.text }}
        {% if item|has_children:request.user %}
        <ul>
            {% render_menu item top top_ %}
        </ul>
        {% endif %}
    </li>
    {% endfor %}

Without this filter and if using `{% if item.menu_set.all %}`, we get empty **<ul>** in the html code for users who do not have permission to see submenus.

Also, fixed a bug in a get_template.