rendered_markdown: Rework CSS for markdown headings.
rendered_markdown: Rework CSS for markdown headings.

Extracted by tabbott from the original pull request, with additional
changes to document the surprising margin-top in our current
implementation and avoid a bit of unnecessary CSS.

eeshangarg/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip