Improve display of "duration" template sensors.

[Olen]

A template sensor with the following settings:

    device_class: duration
    unit_of_measurement: s

Where the value can be high (several days) is not handled very pretty in the UI.

The value is always displayed as "HH:MM:SS", so it can show states such as "123:45:32".
The "display precision" is currently not very useful for this kind of sensor:

Maybe options like these would be better?

Display precision
- days (5 days)
- hours (5 days, 3 hours)
- minutes (5 days, 03:45)
- seconds (5 days, 03:45:32)

Maybe even and "auto" mode that could be smart enough to automatically switch between display units them depending on the actual duration.

if seconds > (60 * 60 * 24 * 7):
  # Display days only for > one week
elif seconds > (60 * 60 * 24):
  # Display days and hours
elif seconds > (60 * 60):
  # Display hours:minutes
else:
  # Display minutes:seconds

• Not sure how to visually differntiate the last two, this was only a quick mockup..

Also, the history graph is... uhm...

Not sure how to fix that, but at least it does not really work as it is with these kind of values...