Unify bottom margins of lists and paragraphs

Only considering bottom margins, we currently have:

    p { margin-bottom: 1em }
    ul { margin-bottom: 0 }
    li { margin-bottom: 0}

However, when a list is followed by a title (which has no `top-margin`), the list and the title are not separated. This pull request changes the bottom margin to `1em`, as is done with paragraphs.