Remarkup: Correctly render inline embed layout

Summary:
The generated HTML is like <p>some text <div …>…</div> more text</p>, and HTML <p/> tags may not contain block content like <div/> tags. Browsers actually parse this as if it was <p>some text </p><div …>…</div> more text<p></p> (sic).

The layout CSS class already has display: inline set, but this is not sufficient. Browser's HTML parser doesn't care what CSS rules will be applied, it only deals with the meanings of tags.

Fixes T7201.

Test Plan:
Verify that the following displays the image inline:

some text {Fnnn,layout=inline} more text

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Projects: #remarkup

Maniphest Tasks: T7201

Differential Revision: https://secure.phabricator.com/D11706