help: 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/templates/help-menu.html

@@ -3,13 +3,14 @@
 {% endcomment %}
 
 {% load i18n %}
+{% load plinth_extras %}
 
 <li id="id_help_menu" class="dropdown nav-item">
   <a href="{% url 'help:index' %}" title="{% trans "Help" %}"
      class="nav-link dropdown-toggle" data-bs-toggle="dropdown"
      role="button" aria-expanded="false" aria-haspopup="true"
      id="id_help_menu_link">
-    <span class="fa fa-question-circle nav-icon"></span>
+    {% icon 'question-circle' %}
     <span class="nav-text d-md-none">
       {% trans "Help" %}
     </span>