Misaligned legend for `margin-caption` figures.

[oscarbranson]

Just updated jupyter-book, and have found all the legends for margin-caption class figures are now aligned so the top of the margin legend is aligned with the bottom of the figure. In previous versions the top of the legend and the figure were aligned, which was a lot clearer.

This is fixed by downgrading jupyter-book to 0.14, but this solution is not ideal.

This seems related to #841, but opening a new issues as that one seems stale.