feat(rxFloatingHeader): Floating header for tables

Closes #619

Major kudos to @glynnis and @Droogans
It's specifically `rxJq` because it's adding in things from jQuery that are not present in jQuery-lite (which is what Angular ships with). The name is short though, and I didn't want to call it rxJquery, as that seemed presumptuous. `rxDOMHelper` might be best, as it doesn't 100% implement what the equivalent methods do in jQuery

And I do have plans to push this out to a stand-alone component in the future.