Add promos to the end of HPR episodes
We should promote other shows and events at the end of HPR.
- Get promos from other podcasts/events/projects/FLOSS .org's
hprtranscodeso that it adds an appropriate promo.
- Check if there is one for that day, week, fortnight
- Otherwise, pick from a randomized list, and then move the chosen one out of the list until they are all played.