use uwsgi's cron feature

I am not very confident with our cron setup, especially when it comes to logging. That is also the major blocker for #120 (closed).

I recently learned that uwsgi has a "cron-like facility" which would simplify things as the example deployment already uses uwsgi.

Then again, I am not really sure how well logging works in this case. Also, most admins are probably more familiar with cron than uwsgi, so I am not sure whether this simplifies things for them.

@muecke what do you think? Could you test this somewhere?