NumberInputWidget: Don't use Math.sign()
NumberInputWidget: Don't use Math.sign()

It's ES6 and not supported by IE11, and we don't seem to have a shim for
it yet. Easy enough to just avoid it.

Bug: T102859
Change-Id: Icfbd6979b3db227d64962ec5d7732525025398e2

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui