Add Mastodon social media channel

Author: philwareham

src/templates/forms/misc/body_footer.txp

@@ -9,8 +9,9 @@
                         <meta itemprop="name" content="Textpattern CMS">
                         <meta itemprop="sameAs" content="https://en.wikipedia.org/wiki/Textpattern">
                         <ul class="social-channels">
-                            <li><a class="twitter" rel="external" itemprop="sameAs" href="https://twitter.com/textpattern" title="Follow us on Twitter">Twitter</a></li>
-                            <li><a class="github" rel="external" itemprop="sameAs" href="https://github.com/textpattern" title="Follow us on GitHub">GitHub</a></li>
+                            <li><a class="github" rel="me external" itemprop="sameAs" href="https://github.com/textpattern" title="Follow us on GitHub">GitHub</a></li>
+                            <li><a class="mastodon" rel="me external" itemprop="sameAs" href="https://indieweb.social/@textpattern" title="Follow us on Mastodon">Mastodon</a></li>
+                            <li><a class="twitter" rel="me external" itemprop="sameAs" href="https://twitter.com/textpattern" title="Follow us on Twitter">Twitter</a></li>
                         </ul>
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                         <p>Your kind donations and sponsorships help us keep Textpattern CMS development alive!</p>