Replacing generic `<div>` with HTML5 elements

The goal being to add semantics to the structure and be more accessible (see [ARIA](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA)):
- `<div id="nav">` becomes `<nav>`
- `<div id="content">` becomes `<main>`
- `<div id="article">` becomes `<article>`
- `<div id="disqus_thread">` becomes `<aside id="disqus_thread">`

:warning: I have no idea how old browsers react to these "new" elements.
Might be worth looking into/testing before merging.

As usual, you can test the new version here: [gl.engestrom.ch](http://gl.engestrom.ch)