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.
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.
Swapped back handling to instead be pre-determined instead of being
based upon session/referrer which would cause inconsistent results when
referrer data was not available (redirect to app-loaded images/files).
To support, this adds a mechansism to provide a URL through request
data.
Also cleaned up some imports in code while making changes.
Closes#4656.
Ensures some sensible text is always in webhook text data.
Also aligned some notification reporting to use centralised activity
system instead of custom success events.
For #4216
