[Bug 862266] Datepickers and preset date ranges for k.Graph.

I added buttons for 1m/3m/6m etc, and changed the date labels into date pickers. The date pickers and the slider update whenever the date range changes, either via the button, the date picker, or the slider.

This doesn't look great, but I think it's ok, and hopefully @rdalal can give it some help.

r?