Use tag icon when tags are used as filter criteria

Use the tag icon rather than using the word "Tag:", in order to be
consistent with other filters.

Change-Id: Ie4731c9b9b2f806184287f829b81afc7b38fd339

src/app/search/template/criteria_tag_item.html

@@ -23,7 +23,7 @@
         </a>
     </div>
     <div class="tag tag-warning" ng-switch-when="Tags">
-        Tag: {{tag.title}}
+        <i class="fa fa-tag"></i> {{tag.title}}
         <a class="tag-remove"
            ng-click="removeTag({tag:tag})">
             &times;