[chores:ui] Switched apply filters button to standard styling

(cherry picked from commit f54b438)

Author: nemesifier

openwisp_utils/admin_theme/templates/admin/change_list.html

@@ -66,7 +66,7 @@ <h3 id="changelist-filter-clear">
             </h3>
           {% endif %}
           {% if cl.filter_specs|length > 4 %}
-            <button id="ow-apply-filter" class="danger-btn">Apply Filters</button>
+            <button id="ow-apply-filter" class="button">Apply Filters</button>
           {% endif %}
         </div>
       </div>
@@ -109,5 +109,5 @@ <h3 id="changelist-filter-clear">
       </div>
     </div>
   </div>
-  
+
 {% endblock %}