luci-base: validate name in firewall zone selector widget

When creating a new zone through the firewall zone selector widget,
validate the given name before creating new items.

Depends on commit 0b6ae96f

 ("luci-base: cbi.js: recognize invalid
input in dropdown create field").

Fixes: #2211
Signed-off-by: Jo-Philipp Wich <jo@mein.io>