Add missing locale key

Author: Perlkonig

public/locales/en/apfront.json

@@ -550,6 +550,7 @@
     "DateEnded": "Date ended",
     "DateStarted": "Date started",
     "CustomCSS": "Customize CSS",
+    "Customize": "Customize",
     "DebugModal": "Debugging information",
     "DefaultDisplay": "Default display",
     "Delete": "Delete",