index: Generalize showing custom shortcut icons

Instead of detecting 'custom' in the name of the icons (which can happen if the
icon basename itself contains the word 'custom'), check if the icon is already
an absolute URL path.

Tests:

- Add a custom shortcut and provide a custom icon for it in
/var/www/plinth/custom/static/icons/. Visit the frontpage and ensure that the
icon is visible.

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

plinth/templates/index.html

@@ -62,7 +62,7 @@
                     <a href="{{ shortcut.url }}">
                   {% endif %}
                         <div class="card-icon">
-                          {% if "custom" in shortcut.icon %}
+                          {% if shortcut.icon|first == "/" %}
                             <img src="{{ shortcut.icon }}"/>
                           {% else %}
                             {% if shortcut.app_id %}