- add "ActivityPub" library to handle server to server federation and basic
  client to server protocols using activitypub:
  - add webfinger endpoint to look for actor
  - add actor definition with inbox / outbox / followers
  - remote follow an actor
  - create notes with possible preview cards
  - interract with favourites, reblogs and replies
  - block incoming actors and/or domains
  - broadcast/schedule activities to fediverse followers using a cron task
- For castopod, the podcast is the actor:
  - overwrite the activitypub library for castopod's specific needs
  - perform basic interactions administrating a podcast to interact with fediverse users:
    - create notes with episode attachment
    - favourite and share a note + reply
    - add specific castopod_namespaces for podcasts and episodes definitions
- overwrite CodeIgniter's Route service to include alternate-content option for
  activitystream requests
- update episod...