Add link to Mastodon account

This adds a social media link to our new mastodon account.

It also adds `rel="me"` to the social media links, which is a semantic hint that these are pages about EmberJS on other sites, and which should cause our Mastodon profile to show as "Verified" as the owner of emberjs.com.

Author: ef4

addon/components/es-footer-info.hbs

@@ -11,7 +11,7 @@
   </div>
   <div class="footer-social hide-on-mobile">
     {{#each @socialLinks as |link|}}
-      <a href={{link.href}} title={{link.title}} aria-label={{link.label}}>
+      <a href={{link.href}} title={{link.title}} aria-label={{link.label}} rel="me">
         {{svg-jar link.class}} {{link.title}}
       </a>
     {{/each}}

addon/constants/es-footer.js

@@ -15,6 +15,11 @@ const socialLinks = [{
   href: 'https://discordapp.com/invite/zT3asNS',
   class: 'discord-logo',
   label: 'Join the Ember Community Discord'
+}, {
+  title: 'Mastodon',
+  href: 'https://hachyderm.io/@emberjs',
+  class: 'mastodon-logo',
+  label: 'Official Ember Mastodon Account'
 }];