Sibling layout interactions in content, like `p+ul`

[gnprice]

Zulip web has a couple of CSS rules that adjust spacing between sibling block elements. From web/styles/rendered_markdown.css:

    /* The spacing between two paragraphs is significantly larger.  We
       arrange things so that this spacing matches the spacing between
       paragraphs in two consecutive 1-line messages. */
    & p + p {
        margin-top: 10px;
    }
// …

    /* Reduce top-margin when a paragraph is followed by an ordered or bulleted list */
    & p + ul,
    p + ol {
        margin-top: 0;
    }

(The details have changed occasionally over the years, as in zulip/zulip@d7aa186dabb8587d48c1ce076c92b049e4fbba02 and zulip/zulip@62f2396ee27f132a25f906107b60fa19b35c97ab.)

We should either match that behavior, or determine there's a good reason for mobile to differ from web here.