switch font size setting unit to REM

@nicol this switches the unit for font size to REM. However it's a bit janky because there don't seem to good UI helpers for numeric settings that aren't Integers. this will validate the input on save, but if it's wrong you lose any other changes you made on the page at the same time.

So I would suugest to stick with px for now, until we have a more bespoke settings interface.

But it's here if you have a strong preference.