Features/add more undocumented env vars

[jessebot]

Adds the following new parameters (found either directly in the .env.docker file or in the admin interface):

pixelfed:
  filesystem:
    # -- Many applications store files both locally and in the cloud.
    # For this reason, you may specify a default “cloud” driver here.
    # This driver will be bound as the Cloud disk implementation in the container.
    cloud: "s3"
    driver: "local"

  covid:
    enable_label: true
    label_url: "https://www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public"
    label_org: "visit the WHO website"

  # -- not entirely sure if this is a list of banned usernames or text to
  # display instead of banned usernames
  banned_usernames: ""

  # -- delete local files after saving to the cloud
  media_delete_local_after_cloud: true

  # horizon - for interfacing with redis
  horizon:
    # -- prefix will be used when storing all Horizon data in Redis
    prefix: "horizon-"
    # -- darkmode for the web interface in the admin panel
    dark_mode: false

  # -- max size for custom emojis, in... bytes?
  custom_emoji_max_size: 2000000

  # -- types of media to allow
  media_types: "image/jpeg,image/png,image/gif"

  # -- exp loops (as in loops video? 🤷
  exp_loops: false

  # -- library to process images. options: "gd" (default), "imagick"
  image_driver: "gd"

  instance:
    # -- instance contact max per day
    contact_max_per_day: ""

  activitypub:
    # activity pub logger?
    logger_enabled: false