Replace the references to gitter by Discord (#563)

Author: kubukoz

_includes/footer.html

@@ -12,7 +12,7 @@ <h4>LEARN</h4>
                 <h4>COMMUNITY</h4>
 
                 <div class="hline-w"></div>
-                <a class="grey-text text-lighten-3" href="https://gitter.im/scala-js/scala-js">Gitter chat</a><br/>
+                <a class="grey-text text-lighten-3" href="https://discord.com/invite/scala">#scala-js on Discord</a><br/>
                 <a class="grey-text text-lighten-3" href="http://stackoverflow.com/questions/tagged/scala.js">Stackoverflow</a><br/>
                 <a class="grey-text text-lighten-3" href="https://groups.google.com/forum/?fromgroups#!forum/scala-js">Mailing list</a><br/>
                 <a class="grey-text text-lighten-3" href="http://twitter.com/scala_js">Twitter <i aria-hidden="true" class="fa fa-twitter"></i></a><br/>

community/index.md

@@ -11,10 +11,10 @@ your issues, please use the following resources.
 Many developers have probably faced the same issues you have, so there might be an answer on Stack Overflow just
 waiting to be read.
 
-### [Gitter chat room](https://gitter.im/scala-js/scala-js)
+### [`#scala-js` on Discord](https://discord.com/invite/scala)
 
-For fastest answers, join our [Scala.js Gitter chat room](https://gitter.im/scala-js/scala-js) and ask your questions,
-or just chat with other Scala.js developers! There are currently over 500 users in the chat room.
+For fastest answers, join the `#scala-js` channel in the [official Scala discord](https://discord.com/invite/scala) and ask your questions,
+or just chat with other Scala.js developers!
 
 ### [Official mailing list](https://groups.google.com/forum/?fromgroups#!forum/scala-js)