VisualEditor: add support for Document API

Project VisualEditor support (#49)

added VisualEditor support:
* T02-04 Implement delete feature: update delete api for document to return a 405 error if the document is attached to a module

* JSON Schema: Manage object, array and number types + fix some issues

* JSON schema validation for document REST api + generate automatic slug for document

* fix issue with the JSON schema validation for document REST api

* Manage multiple error messages in the document serializer

* Update the schema example

* Add service to generate preview + fix the update of document to remove content

* Manage 404 error for the generate_snapshot service.

* Update README.md

* remove SessionAuthentication in Django Reest Api to handle auth correctly from outside miller  (with auth bearer)

* enable watchmedo autorestart for docker-compose dev

* Fix the validation of the json field using the JSON schema

* add en_GB for both title and description in the default schema for document payloads.

* bump version number to 2.2.0

Co-authored-by: frederic.reis <frederic.reis@uni.lu>