More robust florin, without negative margin

Florin was introduced because the Times New Roman italic `f` extends
weirdly far on either side, whereas the Times New Roman italic florin
symbol is much more...reserved. Or something. I dunno, it looks bad.
Anyway, the thing with florin is it has tons of spacing on either side
for whatever reason, so we use a negative margin to get it to overlap
normally with adjacent characters.

Which seems pretty natural, but unfortunately keeps causing problems
(#61, #90, now #295):
https://github.com/mathquill/mathquill/issues?labels=Florin&state=closed

I suspect that negative margins on inline elements is uncommon, I don't
think the box model is really meant for inline elements, `width` and
`height` are even ignored on them. Instead, an `inline-block` with a fixed
width narrower than the florin symbol (the whole symbol is still visible
because it defaults `overflow:visible`) and `position:relative` (to overlap
on both sides, not just the right side) seems less likely to cause
layout quirks. Or at least, unique layout quirks, `inline-block`s cause
tons of layout quirks but MathQuill already works around them because
almost everything besides letters and digits are `inline-block`s, and
`position:relative` basically doesn't affect layout.