add dynsel clear button

scipost_django/common/templates/htmx/dynsel.html

 <div class="hx-dynsel">
   <div class="input-query-container">
+    <div role="button"
+         type="button"
+         hx-get="{% url "common:empty" %}"
+         hx-swap="outerHTML"
+         hx-target="next option[selected]">
+      <span class="text-body">&#26;</span>
+    </div>
+
     {% include "django/forms/widgets/select.html" %}
  
     <input name="q"