Review all external DOM utils

[japgolly]

Have a look through all the JsUtil and DomUtil I've got scattered all over the place and see which bits would make more sense here.

[japgolly]

Nothing much for 1.2.0, only #497

There's other stuff for 2.0 but I think sjrd may have already made those changes independently. Revisit this in 2.0

[japgolly]

Nothing new for 2.0