For #194, convert the POTD management command to a celery task. Refactored to put the logic for the command into a function, and the command simply calls this function. The task can also just call this function. Added some basic tests for the new function.

<div class="smiley_farm">
{% for s in smilies %}
<img src="{{ s.image.url }}" alt="{{ s.code }}" title="{{ s.title }} {{ s.code }}" />
{% endfor %}
</div>