Extend usage new MD widget

[chrisvanrun]

A few places where I think it can also be used nicely:

• Email writing (is custom rendering possible without ANY site CSS?)
• Blog post creation
• Page documentation

[jmsmkn]

You might have some luck with ignoring the site CSS by using the all property, see https://developer.mozilla.org/en-US/docs/Web/CSS/all#values

[chrisvanrun]

The blog post has been implemented. Email writing can likely be stripped by using both all mentioned by James + additional bleaching of the class attribute in the md2_email (maybe called slightly differently).

Also re-adding the Helvetica styling found in the email template might be nice. However, all of this is a bit of work and emailing doesn't happen all that often. So we should likely not bother!