api: Fix showing icons for custom shortcuts

Tests:

- Add a custom shortcut and provide a custom icon for it in
/var/www/plinth/custom/static/icons/. Visit the page /plinth/api/1/shortcuts/.
The icon path for the custom shortcut is correct and accessible from browser.

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

plinth/modules/api/views.py

@@ -65,6 +65,9 @@ def _get_icon_url(app_id, icon_name):
     if not icon_name:
         return None
 
+    if icon_name.startswith('/'):
+        return icon_name
+
     if app_id:
         return static(f'{app_id}') + f'/icons/{icon_name}.png'