Content inside HTML tags on their own lines is lost when editing a Markdown page with the WYSIWYG editor

Steps to reproduce:

1. Create a page in the markdown/1.2 syntax with the content:

   before
   
   <div class="red">
   
   *x*
   
   </div>
   
   after
   

2. Edit the page with the WYSIWYG editor and save it without changes

Expected: the content is unchanged.
Actual: the content becomes the following, so the paragraph inside the div and the closing </div> are lost:

before

<div class="red">

after

Cause: each HTML block is parsed as its own html macro (clean=false), so the opening <div class="red"> and the closing </div> become two unbalanced macros, with the Markdown content as normal blocks between them. The WYSIWYG editor can't round-trip an unbalanced <div>.

Possible solution: parse the whole <div>…</div> block, content included, into a single html macro with clean=false, and render that macro back as the <div> block in Markdown.

Workaround (once MARKDOWN-56 is released): add the data-xwiki-group attribute to the div so it is parsed as a Group.