Adding accessibility features

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

_includes/sections/quotes.html

@@ -32,7 +32,7 @@
   <li><a href="https://papers.ssrn.com/sol3/papers.cfm?abstract_id=998565">'I've Got Nothing to Hide' and Other Misunderstandings of Privacy (Daniel J. Solove - San Diego Law Review)</a></li>
 </ul>
 
-<h1 id="quotes" class="anchor"><a href="#quotes"><i class="fas fa-link anchor-icon"></i></a> Quotes</h1>
+<h1 id="quotes" class="anchor"><a href="#quotes" aria-label="Quotes' section"><i aria-hidden="true" class="fas fa-link anchor-icon"></i></a> Quotes</h1>
 
 <blockquote class="blockquote">
   <p>Ultimately, saying that you don't care about privacy because you have nothing to hide is no different from saying you don't care about freedom of speech because you have nothing to say.  Or that you don't care about freedom of the press because you don't like to read. Or that you don't care about freedom of religion because you don't believe in God. Or that you don't care about the freedom to peacably assemble because you're a lazy, antisocial agoraphobe. </p>