Fix margin bug at bottom of lists (Cherry-pick of #218)

[Eric-Arellano]

Fixes #201. As described there, we had two issues:

1. For ordered lists, there was no margin underneath it.
2. For unordered lists, we were adding a margin inside nested list elements, which was not intended. We only want to add a margin at the very end of the outer-most list.

This is fixed by only applying the margin to the elements article.pytorch-article ol and article.pytorch-article ul, which only apply to the outer most list.

Before:

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After:

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