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.

"""
Celery tasks for the POTD app.

"""
from celery.task import task

import potd.tools


@task
def pick_potd():
    potd.tools.pick_potd()