luci-base: reintroduce accidentally lost isCreateable() function

Fixes: FS#2976
Fixes: 711f7592

 ("luci-base: harmonize JS class naming and requesting")
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2976
Signed-off-by: Jo-Philipp Wich <jo@mein.io>