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.

{% load url from future %}
{% load markup %}
{% load smiley_tags %}
{% load bio_tags %}
{% avatar user 0 "left" %}{% profile_link user.username %}:<br />{{ msg|safe }}