Adjust sidenotes to not rely on border width subtraction

[DanilaFe]

Closes #26.

As I outlined in the issue:

The reason you're still seeing the issue is that borders (and borders in particular) are rounded by the browser to device pixel boundaries. Thus, the computation for the [smaller padding to compensate for the bigger border] doesn't exactly match the change in border width. The end result is a barely-noticeable layout shift.

There's no way to soundly fix this since rounding behavior depends on display, pixel density, and zoom. Instead, I can change our style to change the border color and add a drop shadow to highlight the hovered-over note. This precludes the need for the computation I described (because the border width now remains constant), and thus removes any place where rounding can make its way in.

This PR does this.

No Hover	Old Hover	New Hover

I think it looks pretty nice:

Reviewed by @benharsh -- thanks!

Testing

• hover no longer changes line breaks in sidenotes on any zoom level I tried in the 10 myths, part 7 post from the original issue.

[bradcray]

This change looks good to me in action, and I verified that the sidenotes that caused me to open the original issue don't change their linefeeds under this branch. Thanks!