Description editor: preserve newlines on export, toggle Quote, highlight active toolbar buttons

- Enter now starts a new <p> (defaultParagraphSeparator) so line breaks
  in the description box show up as line breaks in HTML/Markdown/Confluence
  exports; htmlToMarkdown also handles <div>-separated paragraphs.
- Clicking Quote while already in a blockquote toggles back to a paragraph.
- Toolbar buttons (Bold, Italic, Bullet, Number, Quote) turn blue to
  reflect the formatting state at the current cursor/selection.

exporters/htmlmd.js

@@ -42,6 +42,8 @@ function htmlToMarkdown(html) {
     .replace(/<hr\s*\/?>/gi, '\n---\n')
     .replace(/<p>/gi, '\n')
     .replace(/<\/p>/gi, '\n')
+    .replace(/<div>/gi, '\n')
+    .replace(/<\/div>/gi, '\n')
     .replace(/<[^>]+>/g, '');
 
   return decodeEntities(out).replace(/[ \t]+\n/g, '\n').replace(/\n{3,}/g, '\n\n').trim();