ui: Use inline SVG icons for internal zone message

Tests:

- In Privoxy app with only internal interface defined, the icon appears as
before in message in light/dark themes.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/templates/internal-zone.html

@@ -3,13 +3,14 @@
 {% endcomment %}
 
 {% load i18n %}
+{% load plinth_extras %}
 
 {% block internal_zone_warning %}
   {% for component in firewall %}
     {% if not component.is_external %}
       <div class="alert alert-info d-flex align-items-center" role="alert">
         <div class="me-2">
-          <span class="fa fa-info-circle" aria-hidden="true"></span>
+          {% icon 'info-circle' %}
           <span class="visually-hidden">{% trans "Info:" %}</span>
         </div>
         <div>