remove twitter link and add title attributes

Author: rettinghaus

_includes/header.html

@@ -9,13 +9,10 @@
     </div>
   </div>
   <div class="navbar-section" id="socialLinks">
-    <a class="twitterLink" target="_blank" href="https://twitter.com/MusicEncoding">
-      <img src="{{site.style_baseurl}}/pix/twitter.png" alt="Twitter" class="logo"/>
-    </a>
-    <a class="githubLink" target="_blank" href="https://github.com/music-encoding">
+    <a class="githubLink" target="_blank" href="https://github.com/music-encoding" title="Link to GitHub">
       <img src="{{site.style_baseurl}}/pix/github.png" alt="GitHub" class="logo"/>
     </a>
-    <a class="slackLink" target="_blank" href="https://music-encoding.slack.com/">
+    <a class="slackLink" target="_blank" href="https://music-encoding.slack.com/" title="MEI Slack channel">
       <img src="{{site.style_baseurl}}/pix/slack.png" alt="Slack" class="logo"/>
     </a>
   </div>