- Adapted existing page picker to be usable elsewhere.
- Added endpoint for getting templates for entity picker.
- Added search template filter to support above.
- Updated book save handling to check/validate submitted template.
- Allows non-visible pages to flow through the save process, if not
being changed.
- Updated page deletes to handle removal of default usage on books.
- Tweaked wording and form styles to suit.
- Updated migration to explicity reflect default value.
Primarily intended to show parent book for chapters when moving/copying
pages, since the default parent selector interfaces, which used the
entity-selector search endpoint, would run this popular query when no
term was present as a default backup.
For #4264
