Updated query classes to align to interface for common aligned
operations.
Extracted repeated string-identifier-based finding from page/chapter
repos to shared higher-level entity queries.
Also moved to a non-static approach, and added a high-level class to
allow easy access to all other entity queries, for use in mixed-entity
scenarios and easier/simpler injection.
Removes page/chpater addSelect global query, to load book slug, and
instead extracts base queries to be managed in new static class, while
updating specific entitiy relation loading to use our more efficient
MixedEntityListLoader where appropriate.
Related to #4823
