luci-app-firewall: inconsistent length checking of firewall zone names

[evlli]

Steps to reproduce:

1. go to: Network → Interfaces → edit $interface → Firewall Settings tab → Assign to new zone with a very long name >11
2. Save & Apply
3. go to: Network → Firewall → edit $new zone name
4. change some setting

Actual behavior:

1. save button does not close the window
2. discard does not discard the changes
3. save & apply will apply the changes anyway

Expected behavior:

no part of the UI complains about long zone names (preferred)
or
no part of the UI lets you set long zone names

[adelton]

This seems like a duplicate of #7522, addressed in #7549.