{% load form_helpers %}

<div class="panel panel-default">
    <div class="panel-heading">
        <strong>Filter</strong>
    </div>
    <div class="panel-body">
        <form action="." method="get" class="form">
            {% for field in filter_form %}
                <div class="form-group">
                    {% if field|widget_type == 'checkboxinput' %}
                        <label for="{{ field.id_for_label }}">{{ field }} {{ field.label }}</label>
                    {% else %}
                        {{ field.label_tag }}
                        {{ field }}
                    {% endif %}
                </div>
            {% endfor %}
            <div class="text-right">
                <button type="submit" class="btn btn-primary">
                    <span class="glyphicon glyphicon-search" aria-hidden="true"></span> Apply filters
                </button>
            </div>
        </form>
    </div>
</div>

{% load form_helpers %}

<div class="panel panel-default">
    <div class="panel-heading">
        <span class="glyphicon glyphicon-filter" aria-hidden="true"></span> 
        <strong>Filter</strong>
    </div>
    <div class="panel-body">
        <form action="." method="get" class="form">
            {% for field in filter_form %}
                <div class="form-group">
                    {% if field|widget_type == 'checkboxinput' %}
                        <label for="{{ field.id_for_label }}">{{ field }} {{ field.label }}</label>
                    {% else %}
                        {{ field.label_tag }}
                        {{ field }}
                    {% endif %}
                </div>
            {% endfor %}
            <div class="text-right">
                <button type="submit" class="btn btn-primary">
                    <span class="glyphicon glyphicon-search" aria-hidden="true"></span> Apply filters
                </button>
            </div>
        </form>
    </div>
</div>

Add 'filter' glyphicon to filter panel header
Add 'filter' glyphicon to filter panel header