Media firewall 3: Introduce media subhashes

[lukasbestle]

Description

Summary of changes

• Inside the media hash directory (/media/site/abcdefghij-1234567890), there is now another directory layer (e.g. /media/site/abcdefghij-1234567890/bcdefghija) to separate all individual files (the original and each thumb)
• Transitional code is added so that all old media URLs stay valid until v6 (these are automatically redirected to the new URLs)
• Existing media files and thumbs are automatically migrated to their new locations to avoid having to regenerate them

Reasoning

• Prevent access to file versions (especially originals but also different thumb variants) by guessing the media URL (so far if you e.g. had /media/site/abcdefghij-1234567890/file-120x.jpg you could change the filename to file.jpg to access the original, which may not be intended by devs).
• Using another directory layer makes sure that the filenames stay clean and don't need to include generated hashes.

Additional context

With the transitional and migration code, there shouldn't be a breaking impact to sites. But since there is already a lot going on in v5, I suggest we include this PR in 5.1 or 5.x. This ensures that users upgrading to v5 with a lot of media files and thumbs don't immediately run into performance problems when all those files are migrated.

Changelog

Enhancements

• It is no longer possible to guess other URLs to file versions (uploaded file originals and their thumbnails) from a thumbnail URL.

Deprecated

• The URL structure of media files (file URLs and thumb URLs) has changed. The old URL structure is still supported for now, but will be dropped in Kirby 6.

Breaking changes

None

Docs

None

Ready?

• In-code documentation (wherever needed)
• More robust migration code (currently not safe against race conditions when multiple requests migrate the same files at the same time)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion