Decided it's relevant to entity updated_at since tags are now indexed
alongside content.
- Also fixed tags not applied on shelf.
- Also enforced proper page API update validation.
- Adds tests to cover.
For #3319Fixes#3370
- Fixed page editor default focus not working as expected due to
misnamed attribute.
- Added owned_by to relevant areas of the API including the docs.
- Made book relation on page accessible even if deleted since it could cause an issue on views, such as audit trail, when the relation is accessed when the book is deleted.
