Add mastodon link.

Author: m-ou-se

static/images/mastodon.svg

@@ -31,6 +31,7 @@
       <div class="flex flex-column mw8 w-100 measure-wide-l pv2 pv5-m pv2-ns ph4-m ph4-l">
         <h4>{{fluent "footer-social"}}</h4>
         <div class="flex flex-row flex-wrap items-center">
+          <a rel="me" href="https://social.rust-lang.org/@rust" target="_blank"><img src="/static/images/mastodon.svg" alt="mastodon logo" /></a>
           <a href="https://twitter.com/rustlang" target="_blank"><img src="/static/images/twitter.svg" alt="twitter logo" title="{{fluent "footer-youtube-alt"}}"/></a>
           <a href="https://www.youtube.com/channel/UCaYhcUwRBNscFNUKTjgPFiA" target="_blank"><img class="pv2" src="/static/images/youtube.svg" alt="{{fluent "footer-alt-youtube"}}" title="YouTube"/></a>
           <a href="https://discord.gg/rust-lang" target="_blank"><img src="/static/images/discord.svg" alt="discord logo" title="{{fluent "footer-discord-alt"}}"/></a>