Fix twitter-view once and for all!

One image URL was broken and it was accidentally using the avatar
directive, so the class has been changed to 'twitter-avatar' and made
explicit.

static/directives/twitter-view.html

@@ -4,7 +4,7 @@
   </p>
   <div class="attribute">
     <span class="info-wrap">
-      <img ng-src="/static/img/default-twitter.png">
+      <span class="twitter-avatar"><img ng-src="{{ avatarUrl }}" fallback-src="/static/img/default-twitter.png"></span>
       <span class="info">
         <span class="author">{{ authorName }} (@{{authorUser}})</span>
         <a class="reference" ng-href="{{ messageUrl }}">{{ messageDate }}</a>