[enh] reset default preferences

searx/templates/courgette/preferences.html

     </p>
 
     <input type="submit" value="{{ _('save') }}" />
+    <div class="right preferences_back"><a href="{{ url_for('clear_cookies') }}">{{ _('Reset defaults') }}</a></div>
     <div class="right preferences_back"><a href="{{ url_for('index') }}">{{ _('back') }}</a></div>
-    </form>    
+    </form>
 </div>
 {% endblock %}

searx/templates/default/preferences.html

     </p>
 
     <input type="submit" value="{{ _('save') }}" />
+    <div class="{% if rtl %}left{% else %}right{% endif %} preferences_back"><a href="{{ url_for('clear_cookies') }}">{{ _('Reset defaults') }}</a></div>
     <div class="{% if rtl %}left{% else %}right{% endif %} preferences_back"><a href="{{ url_for('index') }}">{{ _('back') }}</a></div>
-    </form>    
+    </form>
 </div>
 {% endblock %}

searx/templates/oscar/preferences.html

 
     <input type="submit" class="btn btn-primary" value="{{ _('save') }}" />
 	<a href="{{ url_for('index') }}"><div class="btn btn-default">{{ _('back') }}</div></a>
+	<a href="{{ url_for('clear_cookies') }}"><div class="btn btn-default">{{ _('Reset defaults') }}</div></a>
     </form>
 </div>
 {% endblock %}

searx/webapp.py

                                mimetype='image/vnd.microsoft.icon')
 
 
+@app.route('/clear_cookies')
+def clear_cookies():
+    resp = make_response(redirect(url_for('index')))
+    for cookie_name in request.cookies:
+        resp.delete_cookie(cookie_name)
+    return resp
+
+
 def run():
     app.run(
         debug=settings['server']['debug'],