Always use default twitter image for twitter-view.

It was generating a very low resolution (16x16) avatar for the null user
and blowing it up to fit. It's much better to just use what would've
been the fallback.

static/directives/twitter-view.html

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