- 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.
Added due to now not being able to perform an exact search where
contains a trailing backslash.
Now all backslashes in exact terms are consided escape chars
and require escaping themselves.
Potential breaking change due to search syntax handling change.
Related to #4535.
Also prevented use of empty exact matches.
Prevents issues when attempting to use exact search terms in inputs for
just search terms, and use of single " chars within search terms since
these would get auto-promoted to exacts.
For #4535
