note: this will likely break things for people in the same way that doing
this to tooltip did… however we think this is a better insertion model for
z-index,scrolling,modal,etc. applications

* Fix as documented here: https://github.com/twitter/bootstrap/issues/4756#issuecomment-9952781
* Stops propagation for touchstart for dropdowns

remove modal-open class
hopefully resolve z-index headaches

only stop prop on form – as people seem to be relying on dropdown hiding when clicking internal elements