ui: Use inline SVG icons for app's service-not-running message

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/templates/app.html

@@ -27,7 +27,7 @@
       <div id='service-not-running' role="alert"
            class="alert alert-danger d-flex align-items-center {{ is_enabled|yesno:',d-none' }}">
         <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>