Made the theme button toggable by keyboard
Added aria-label to many <a> without any text (usually, they contains Font Awesome icons)
Added aria-hidden whenever necessary
Labelled the "Copy URL and Description" contained in footer
Added an hidden label for the menu toggle for mobile accessibility
Changed some colors to meet the Color Contrast AA level: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
JavaScript was checking if element clicked is exactly .nav-summary,
which isn't a problem when just clicking text inside it, but if we try
to click another element inside like <span>, it won't work, since now
this span is clicked, not .nav-summary.
Closes#1549
