Just as a draft for prototyping and playing around to get things
started.

Rolled out HTML editor field and store logic across all target entity
types. Cleaned up WYSIWYG input logic and design.
Cleaned up some injected classes while there.

For book, shelves and chapters.
Made much of the existing handling generic to entity types.
Added new MixedEntityListLoader to help load lists somewhat efficiently.
Only manually tested so far.

So that multiple elements on the page can share the same popup, with
different search options.

To ensure consistenent behaviour before/after changes.
Added tests to cover.

Also aligned books, shelves and chapters to return description content
and some relations (where not breaking API) in create/update responses
also so that information can be seen direct from that input in a
request.

API docs and tests not yet updated to match.

Fixed issue where an existing description_html field would not be
updated via 'description' input.