Changes 10: New MemoryContentStorageHandler

[bastianallgeier]

This PR …

• 🚨 Merge first: Changes 1: Refactor content storage to use new VersionId class #6436
• 🚨 Merge first: Changes 2: Plain Text Handler Refactoring #6439
• 🚨 Merge first: Changes 3: New Version class #6442
• 🚨 Merge first: Changes 4: Refactor PlainTextContentStorageHandler to use Language object #6448
• 🚨 Merge first: Changes 5: ContentStorageHandler Abstract #6449
• 🚨 Merge first: Changes 6: Version class improvements and unit tests #6450
• 🚨 Merge first: Changes 7: Move ContentStorage methods to ContentStorageHandler #6454
• 🚨 Merge first: Changes 8: New ModelWithContent::version method #6455
• 🚨 Merge first: Changes 9: Remove content storage class #6456

Features

• New MemoryContentStorageHandler class (see outlook for details what this class is supposed to do)

Refactor

• Move ::write and ::ensure into ContentStorageHandler class and make them abstract
• Turn ContentStorageHandler::update and ContentStorageHandler::create into non-abstract methods that use ::write under the hood.
• Use ContentStorageHandler::ensure in Version::ensure

Outlook

• The new MemoryContentStorageHandler class will be used in ModelWithContent::setContent and ModelWithContent::setTranslation. It will replace the PlainTextContentStorageHandler instance with an in-memory version of the content or translation that is passed via array. This will (hopefully) keep our virtual pages, files and users working, while still making use of the underlying storage implementation. It should also help to untie the content vs. translations knot.

Breaking changes

None

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and checks all pass

For review team

• Add changes & docs to release notes draft in Notion