add domId and generateDomId to the Html helper

Using a single helper for these two functions (it should only be one
with the array-injection weirdness of domId being a different function)
means there is only one helper, not all helpers, responsible for
generating dom identifiers.
ask the Html helper for dom ids

Possibly refactor this given that only _inputLabel calls domId with an
array...
add a bc wrapper for the existing domId function

it's quite possible that user-land code is calling this function on
random helpers, make sure it still works