luci-base: ui.js: warn about connectivity loss on changing iface settings

If specific settings such as the protocol, IP address or netmask of an
interface the user is connected to are changed, the apply/rollback
mechanism might interfere. Display an additional warning dialog in this
case, instructing the user to manually reconnect and offering to continue
with a less safe unchecked apply mechanism.

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