fix(oscar,preferences): use base_url for prefix of sharing 'currenly saved preferences'

searx/templates/oscar/preferences.html

         {{ _("These cookies serve your sole convenience, we don't use these cookies to track you.") }}
         </p>
         <p style="margin:20px 0;">{{ _('Search URL of the currently saved preferences') }} <small class="text-muted">({{ _('Note: specifying custom settings in the search URL can reduce privacy by leaking data to the clicked result sites.') }})</small>:<br/>
-          <input readonly="" class="form-control select-all-on-click cursor-text" type="url" value="{{ url_for('index', _external=True) }}?preferences={{ preferences_url_params|e }}{% raw %}&amp;q=%s{% endraw %}">
+          <input readonly="" class="form-control select-all-on-click cursor-text" type="url" value="{{ base_url }}?preferences={{ preferences_url_params|e }}{% raw %}&amp;q=%s{% endraw %}">
         </p>
 
         <input type="submit" class="btn btn-primary" value="{{ _('save') }}" />

searx/templates/pix-art/preferences.html

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