base: Add link to about for unauthenticated users

Since the about page is now public, adding a link to it in the navbar allows the
users to easily discover it and find out about FreedomBox.

Tests:

- In mobile view, the hamburger menu show the icon and text. Link works.

- In desktop mode, only the icon is visible. Link works.

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

plinth/templates/base.html

@@ -206,6 +206,15 @@
                     </span>
                   </a>
                 </li>
+                <li class="nav-item">
+                  <a class="nav-link" href="{% url 'help:about' %}"
+                     title="{% trans "About" %}">
+                    <span class="fa fa-question-circle nav-icon"></span>
+                    <span class="nav-text d-inline-block d-md-none">
+                      {% trans "About" %}
+                    </span>
+                  </a>
+                </li>
                 <li class="nav-item">
                   <a class="nav-link" href="{% url 'users:login' %}"
                      title="{% trans "Log in" %}">