Added extendable/scalable formatter for webhook data

Added extendable/scalable formatter for webhook data

Creates a new organsied formatting system for webhook data, with
interfaces for extending with custom model formatting rules.
Allows easy usage & extension of the default bookstack formatting
behaviour when customizing webhook events via theme system, and keeps
default data customizations organised.

This also makes the following webhook data changes:
- owned_by/created_by/updated_by user details are loaded for events with
  Entity details. (POTENTIALLY BREAKING CHANGE).
- current_revision details are loaded for page update/create events.

Added testing to cover added model formatting rules.

For #3279 and #3218