luci-app-attendedsysupgrade: rework check-for-new-firmware

luci-app-attendedsysupgrade: rework check-for-new-firmware

Do a complete overhaul of the firmware check:

When the Status -> Overview page is loaded, we check if the
configuration is set.  If not, we ask the user to choose between
enabled or disabled.  Once this is done, it never appears again
(much like the "set password" logic in the shell).

As a result, there is no longer a persistent section on the Overview
page with a simple toggle eating real estate and playing havoc with
the Hide/Show button scheme.

When the setting is enabled, then every time Status -> Overview
is loaded, we do the firmware check and display an alert notice
as before.  But, the alert notice now contains a button to disable
the alerts, so navigation is still simple, you don't have to dig
around to figure out how to turn it off.

The logic for version comparisons was cleaned up and simplified.

Fixes: #7925
Fixes: #8226
Signed-off-by: Eric Fahlgren <redacted>