luci-app-nut: update monitor for package refactor

luci-app-nut: update monitor for package refactor

https://github.com/openwrt/packages/pull/28875 deals with
https://github.com/openwrt/luci/pull/8420#issuecomment-4071252681
and
https://github.com/openwrt/packages/pull/28875#issuecomment-4079307540

by simplifying the configuration of notification messages and flags.

At the same time it modernizes the monitoring user configuration.

Supersedes #8434

This commit syncs luci-app-nut for the nut package changes in
https://github.com/openwrt/packages/pull/28875 .

Signed-off-by: Daniel F. Dickinson <redacted>