bookend: Only show bookend buttons in trailing bookends.

Apparently, this template was just always missing this check.

Fixes #5237.

static/templates/bookend.handlebars

@@ -3,6 +3,7 @@
 {{#if bookend_content}}
 <div class="{{#if trailing}}trailing_bookend{{/if}} bookend sub-unsub-message">
     <span>{{bookend_content}}</span>
+    {{#if trailing}}
     <div class="sub_button_row">
         <button class="btn stream_sub_unsub_button btn-default" type="button" name="subscription">
             {{#if subscribed}}
@@ -12,5 +13,6 @@
             {{/if}}
         </button>
     </div>
+    {{/if}}
 </div>
 {{/if}}