Default templates: Added page picker and working forms

- 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.

app/Entities/Tools/TrashCan.php

@@ -202,6 +202,10 @@ class TrashCan
             $attachmentService->deleteFile($attachment);
         }
 
+        // Remove book template usages
+        Book::query()->where('default_template', '=', $page->id)
+            ->update(['default_template' => null]);
+
         $page->forceDelete();
 
         return 1;