Various accessibility-minded fixes

- added aria-hidden="true" to font icons and the "<3" ASCII art
- github and twitter font icon links now actually have some real
(hidden) text that is announced by AT, not just empty links anymore
- using <label> to wrap <select> has quirky results in AT - some screen
readers end up reading out all options in sequence - swapped it for a
<div> and injected an aria-label to the <select> (this *could* also be
done with a title attribute - though that may generate tooltips - or an
explicit hidden <label for="..."> but requires an id on the <select> -
using aria-label is the quickest, least intrusive way)
- added aria-label to file browser (though AT support is a bit flaky in
general for this, depending on browser/AT/OS combination)