Improve support for Unlimited plans

As the extension was calculating the used GB from the Limit and Remaining, it didn't show any usage on Unlimited plans (as that would be 0 minus 0).

I've updated the extension to grab the Usage from Snap's page rather than calculating it, and also hide fields not relevant on Unlimited plans, as well as the percentage bar.

Haven't removed the bar from the icon canvas, as I felt it still served to illustrate that the extension is a usage meter.