fix: set selected subnav item for icon sets edit template

Author: bhlindemann

src/templates/icon-sets/edit.twig

@@ -9,6 +9,7 @@
 {% import "_includes/forms" as forms %}
 
 {% set fullPageForm = true %}
+{% set selectedSubnavItem = 'icon-sets' %}
 
 {% set title = isNew ? 'New Icon Set'|t('icon-manager') : iconSet.name %}