As per PR #5800
* DB: Planned out new entity table format via migrations
* DB: Created entity migration logic
Made some other tweaks/fixes while testing.
* DB: Added change of entity relation columns to suit new entities table
* DB: Got most view queries working for new structure
* Entities: Started logic change to new structure
Updated base entity class, and worked through BaseRepo.
Need to go through other repos next.
Removed a couple of redundant interfaces as part of this since we can
move the logic onto the shared ContainerData model as needed.
* Entities: Been through repos to update for new format
* Entities: Updated repos to act on refreshed clones
Changes to core entity models are now done on clones to ensure clean
state before save, and those clones are returned back if changes are
needed after that action.
* Entities: Updated model classes & relations for changes
* Entities: Changed from *Data to a common "contents" system
Added smart loading from builder instances which should hydrate with
"contents()" loaded via join, while keeping the core model original.
* Entities: Moved entity description/covers to own non-model classes
Added back some interfaces.
* Entities: Removed use of contents system for data access
* Entities: Got most queries back to working order
* Entities: Reverted back to data from contents, fixed various issues
* Entities: Started addressing issues from tests
* Entities: Addressed further tests/issues
* Entities: Been through tests to get all passing in dev
Fixed issues and needed test changes along the way.
* Entities: Addressed phpstan errors
* Entities: Reviewed TODO notes
* Entities: Ensured book/shelf relation data removed on destroy
* Entities: Been through API responses & adjusted field visibility
* Entities: Added type index to massively improve query speed
Updated the back button to be a proper link instead of a reference to
the last viewed URL since it could break if the last page was the
current one (On validation for example).
Includes test to cover.
Also applied some styleCI changes.
Fixes#2834
Done a little reorganisation while there of misplaced tests.
Moved MarkdownTest to a new PageEditorTest to avoid confusion with
other markdown elements and to align with other page tests.
