Changes 16: ModelWithContent simplifications

[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
• 🚨 Merge first: Changes 10: New MemoryContentStorageHandler #6457
• 🚨 Merge first: Changes 11: Language::ensure & Languages::ensure #6483
• 🚨 Merge first: Changes 12: More Version class improvements #6490
• 🚨 Merge first: Changes 13: New translation classes #6491
• 🚨 Merge first: Changes 14: Model prop fixes #6499
• 🚨 Merge first: Changes 15: More ContentStorageHandler class methods #6500

Reasoning

I'm trying to already use the improvements and changes from the last PRs in ModelWithContent without taking too many steps at once. The full conversion will be still a lot of complicated, potentially breaking steps. Taking it easy will help to keep our mental load in check.

Refactoring

• Simplify ModelWithContent::writeContent by using the new Version::save() method.
• Simplify ModelWithContent::content by using Language::ensure() and Version::content()

Breaking changes

None expected

Ready?

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

For review team

• Add changes & docs to release notes draft in Notion