Add semantic markup to layouts

Re: https://github.com/biolithic/backdroplayouts/issues/29

We definitely need to get a <main> tag back in, one on every page.

Now that I'm looking into the <section> tag again, we might be better off with <div>s. A section should have a headline, and is part of the document outline, neither of which are a given in how these layouts might be used.

Same goes for <aside>... it'd be nice to have one, but we might be better leaving that off since we're not sure how regions will be used.