feat(fediverse): implement activitypub protocols + update user interface

- 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 episode publication logic:
  - remove publication inputs in create / edit episode form
  - publish / schedule or unpublish an episode after creation
  - the podcaster publishes a note when publishing an episode
- Javascript / Typescript modules:
  - fix Dropdown.ts to keep dropdown menu in foreground
  - add Modal.ts for funding links modal
  - add Toggler.ts to toggle various css states in ui
- User Interface:
  - update tailwindcss to v2
  - use castopod's pine and rose colors
  - update public layout to a 3 column layout
  - add pages in public for podcast activity, episode list and notes
  - update episode page to include linked notes
  - remove previous and next episodes from episode pages
  - show different public views depending on whether user is authenticated or not
  - use Kumbh Sans and Montserrat fonts
- update CodeIgniter's config files
- with CodeIgniter's new requirements, update docker environments are now based on
  php v7.3 image
- move Image entity to Libraries
- update composer and npm packages to latest versions

closes #69 #65 #85, fixes #51 #91 #92 #88