power: Use inline SVG icons

Tests:

- All the icons appear as before in both light/dark themes.

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

plinth/modules/power/templates/power_busy_warning.html

@@ -3,11 +3,12 @@
 {% endcomment %}
 
 {% load i18n %}
+{% load plinth_extras %}
 
 {% if pkg_manager_is_busy %}
   <div class="alert alert-danger d-flex align-items-center">
     <div class="me-2">
-      <span class="fa fa-exclamation-triangle" aria-hidden="true"></span>
+      {% icon 'exclamation-triangle' %}
       <span class="visually-hidden">{% trans "Caution:" %}</span>
     </div>
     <div>