Changes:
- Reintegrating work from the [COMRADES](https://www.comrades-project.eu/) project. In particular
  - More complete webhooks support
  - API support for pushing partial updates to posts
  - See #1665 for detailed commits
- Centralised media, ratelimiter, and cdn configuration in `.env` file rather than `application/config` #2454
- Fix validation of category permissions #2486 #2481

Migration info:
- If you have customized configuration for media, ratelimiter or cdn we recommend moving this configuration to the `.env` file. New `.env` params are
    - `MEDIA_MAX_UPLOAD` - maximum file upload size in bytes
    - `RATELIMITER_CACHE` - type of ratelimiter to use. See `application/config/ratelimiter.php`.
    - New CDN params - see `application/config/cdn.php`