Make use of divs and sections consistent

[ccschmitz]

In a few places #primary is a <div>, and in others it's a <section>. This commit moves them all over to <section>'s, which is probably more appropriate.

[BFTrick]

+1 to this. Converting the #primary div into a section makes sense.

[obenland]

As far as I'm concerned, we already use sections consistently. From the spec:

A section, [...] is a thematic grouping of content, typically with a heading.

The section element is not a generic container element. When an element is needed only for styling purposes or as a convenience for scripting, authors are encouraged to use the div element instead. A general rule is that the section element is appropriate only if the element's contents would be listed explicitly in the document's outline.

[obenland]

No action in almost 4 weeks. Let's open a new ticket if there is disagreement with the direction _s takes on this, after #172 made it in.