Make lesson section headers into real header (H2) tags

The sections of a lesson (e.g. prerequisites, steps, etc.) are currently getting printed out as divs, when they really should be header tags IMO. At the very least they need to styled so that they stand out more than the HTML H tags that can be used in the body.

The lesson title is an H1 tag, so ideally the section headers would be H2 tags. That would allow us to use H3 tags for headers within the sections that are delineated by fields.