Improve accessibility of configuration page

[ssbarnea]

First, thanks for maintaining this, I know how painful it can be...

I spotted few quirks which could probably be improved to make it less confusing to setup:

• "Battery Runtime Left to initiate Shutdown" does not mention unit of measure, I suppose is seconds but guess should not be needed
• Most Config options are disabled while NUT is running and there is no text in the UI explaining you that you need to stop it to start reconfiguring it. I probably spent 10 minutes figuring it out, including a lot of google searches.
• "UPS Battery Runtime Measurement Unit" inside Display section apparently has no effect anywhere in the UI, I still see the "12:34:00" as runtime left on top. Later I figured out that this affects NUT details: battery.runtime and likely battery.runtime.low values.