Replace Twitter with Mastodon in footer (#2265)

* add mastodon link to footer, replacing twitter.

README.md

@@ -24,4 +24,4 @@ There are many different ways to contribute to MapRoulette!
 
 * Ask a question about MapRoulette on the [OSM community forum](https://community.openstreetmap.org/tag/maproulette)
 * Join the #maproulette channel on the [OSM US Slack](https://slack.openstreetmap.us/)
-* Follow MapRoulette [on Mastodon](https://en.osm.town/@MapRoulette) or [on Twitter](https://twitter.com/maproulette)
+* Follow MapRoulette [on Mastodon](https://en.osm.town/@MapRoulette)

public/index.html

@@ -10,9 +10,6 @@
     <meta property="og:type" content="website" />
     <meta property="og:title" content="MapRoulette" />
     <meta property="og:description" content="Be an instant contributor to the world’s maps." />
-    <meta property="twitter:title" content="MapRoulette" />
-    <meta property="twitter:description" content="Be an instant contributor to the world’s maps." />
-    <meta property="twitter:image" content="https://openstreetmap.us/img/pages/maproulette/sign.png" />
     <!--
       manifest.json provides metadata used when your web app is added to the
       homescreen on Android. See https://developers.google.com/web/fundamentals/engage-and-retain/web-app-manifest/

src/components/Footer/Footer.js

@@ -103,13 +103,13 @@ class Footer extends Component {
                   <FormattedMessage {...messages.followUs} />
                 </h3>
                 <a
-                  href="https://twitter.com/maproulette"
+                  href="https://en.osm.town/@MapRoulette"
                   target="_blank"
                   rel="noopener noreferrer"
                   className="mr-inline-flex mr-items-center"
                 >
                   <SvgSymbol
-                    sym="icon-twitter"
+                    sym="icon-mastodon"
                     viewBox="0 0 30 24"
                     className="mr-w-6 mr-h-auto mr-fill-twitter"
                   />