Callout rendering is messy

[noelwelsh]

Callouts render differently (and poorly) in PDF and HTML. In PDF they have two titles, the title assigned to the Latex environment, and then the title given to any content.

In HTML they only have two level heading numbering, which is confusing as they are often within at least two levels of headings already.

[noelwelsh]

Proposed fix:

• remove frametitle attribute from InfoCallout, WarningCallout, and DangerCallout environments.
• the incorrect numbering in the HTML is, as far as I can tell, Pandoc messing up. Propose marking callout headings as unnumbered in our style guide.

[d6y]

If it's the problem I think it is, we've worked around this in Essential Slick by just using bold:

<div class="callout callout-info">
**Group By True**

There's a `groupBy { _ => true}` trick 
...

This renders in HTML as: