New sliders

This PR replaces the jslider library with ion.rangeSlider. Examples at: https://winston.shinyapps.io/sliders2/

A couple things to note:

* The handles appear as squares instead of circles in IE8.
* In Safari and the RStudio viewer (tested on Mac and Linux), the sliders jump each time they're updated from the server. This is due to an issue in ion.rangeSlider. It can be seen in https://winston.shinyapps.io/setinput/
* The old sliders are still available in the shinyBootstrap2 package.