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 and broadcast notes with possible preview cards
  • interract with favourites, reblogs and replies
  • block incoming actors and/or domains --> 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 attachements
    • 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

• update CodeIgniter's config files

• with CodeIgniter's new requirements, update docker environments to be based on php v7.3 image

• move Image entity to Libraries

• update composer and npm packages to latest versions

closes #69 (closed) #65 (closed), fixes #51 (closed)