[Enhancement] Improve notification system

[RemiRigal]

The current notification system can result in a lot of notifications for people having a lot of users on their servers.

Possible improvements:

• As suggested in #26, notifications for newly added episodes should not be sent per-user.
• Improve the configuration and add Apprise configs for specific users, something like this:

  notifications:
  enable: true
  apprise_configs:
  - "spush://apikey"
  - urls:
      - "discord://webhook_id/webhook_token"
      - "pover://user@token"
    users:
      - "MyUsername"
      - "MyUser1"
  - urls:
      - "gotify://hostname/token"
    users:
      - "MyUser2"

I'm open to feedback and suggestions !