Implement h1-h5

[sirpengi]

Fixes: #192

An unresolved issue in this PR is that headers at the beginning of a message should not have a top margin. See from
https://github.com/zulip/zulip/blob/main/web/styles/rendered_markdown.css :

    /* Headings: Ensure that messages that start with a heading don't have
       a weirdly blank area at the very start of the message. */
    & h1:first-child,
    h2:first-child,
    h3:first-child,
    h4:first-child,
    h5:first-child,
    h6:first-child {
        margin-top: 0;
    }

We have an open issue #162 to handle content layout where this should be addressed in the future.

Screenshot from android:

[gnprice]

Thanks @sirpengi! Generally this looks good; one nit below, which I'll fix and then I'll merge.

An unresolved issue in this PR is that headers at the beginning of a message should not have a top margin. […] We have an open issue #162 to handle content layout where this should be addressed in the future.

Yeah, thanks for flagging this. I've just added a note about it on that issue: #162 (comment)

[gnprice]

nit: just "Heading", so it agrees with the relevant identifier for the code under test

(Could also be "HeadingNode" — this test file has waffled on whether to name the tests after the widget classes that implement the tested features, or the ContentNode classes that represent them and that the widgets consume.)