Remove extra / in url_for parameter for oscar theme

The `filename` parameter of the `url_for` function doesn't need a leading `/`, or else the resulting URL features a double-slash `//` that throws off searx 0.12.0 with Apache 2.4.25 on Debian, resulting in missing favicons.

searx/templates/oscar/macros.html

 
 <!-- Draw favicon -->
 {% macro draw_favicon(favicon) -%}
-    <img width="32" height="32" class="favicon" src="{{ url_for('static', filename='/themes/oscar/img/icons/' + favicon + '.png') }}" alt="{{ favicon }}" />
+    <img width="32" height="32" class="favicon" src="{{ url_for('static', filename='themes/oscar/img/icons/' + favicon + '.png') }}" alt="{{ favicon }}" />
 {%- endmacro %}
 
 {%- macro result_link(url, title, classes='') -%}