Add return link to thread view

When you're looking at a post, there is now a "return" link that will
take you back to page 1 of the board.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/templates/board/post_detail.html

@@ -39,6 +39,8 @@
     </div>
     <div class="column">&nbsp;</div>
 </div>
+{# return links #}
+[ <a href="{% url 'board:board_detail' board.url %}">{% localize on %}Return{% endlocalize %}</a> ]
 <hr />
 {# posts #}
 <div class="row post">