monitor.go: timezone doesn't account for daylight savings

[Trek Hopton]

When displaying times for sensor data, the times are not correct for zones currently in daylight savings time.

This is because site timezone is represented as a float for the offset from UTC. This offset is used to calculate and display the time but doesn’t account for DST.

[Alan Noble]

To implement this we’d have to move away from using timezone offsets, such as +9.5, to using official timezone location strings “Australia/Adelaide”.

Personally, I think it is okay for site admins to manually change the timezone offset twice a year (assuming they care about DST).

[Trek Hopton]

I don’t think site admins should have to manually change the timezones twice a year.

This is mainly an issue for the monitor page. If the timezones aren’t updated, users will see incorrect times on the value such as temperature and voltage.

We could just have a javascript function that checks if DST applies and modifies the displayed time accordingly.