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/participate.html

@@ -1,5 +1,5 @@
 <div class="page-header">
-  <h1 id="participate" class="anchor"><a href="#participate"><i class="fas fa-link anchor-icon"></i></a> Participate with suggestions and constructive criticism</h1>
+  <h1 id="participate" class="anchor"><a href="#participate" aria-label="Partecipation's section"><i aria-hidden="true" class="fas fa-link anchor-icon"></i></a> Participate with suggestions and constructive criticism</h1>
 </div>
 
 <p>It's important for a website like {{ site.name }} to stay up-to-date. Keep an eye on software updates for the applications listed on our site. Follow recent news about providers that we recommend. We try our best to keep up, but we're not perfect and the internet is changing fast. If you find an error, or you think a provider should not be listed here, or a qualified service provider is missing, or a browser plugin is not the best choice anymore, or anything else... <strong>Talk to us please.</strong> You can also find us on <a href="/services/chat/">several Matrix rooms</a>, primarily <code class="highlighter-rouge">#general:privacytools.io</code>. XMPP users can join there via <a href="https://conversations.im/j/%23general%23privacytools.io@matrix.org">#general#privacytools.io@matrix.org</a>. When using our services, users should follow our <a href="https://wiki.privacytools.io/view/PrivacyTools:Code_of_Conduct">Code of Conduct</a>.</p>