luci-mod-admin-full: auto-migrate ifnames when changing VLAN configuration

luci-mod-admin-full: auto-migrate ifnames when changing VLAN configuration

This change extends the switch VLAN configuration page to automatically
adjust interface ifname options when altering VLAN settings.

For example "eth0" is changed to "eth0.1" when a previously untagged LAN
VLAN is switched to tagged on the CPU port and vice versa.

Notifications are displayed in the page header if an auto migration was
performed.

This change should make the switch configuration more user friendly and
less prone to soft bricking.

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