security: 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/security/templates/security.html

@@ -5,12 +5,13 @@
 
 {% load bootstrap %}
 {% load i18n %}
+{% load plinth_extras %}
 
 {% block status %}
   <div class="btn-toolbar">
     <a class="btn btn-default" role="button" href="{% url 'security:report' %}"
        title="{% trans 'Show security report' %}">
-      <span class="fa fa-line-chart" aria-hidden="true"></span>
+      {% icon 'line-chart' %}
       {% trans "Show security report" %}
     </a>
   </div>