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/Controllers/PageController.php

@@ -279,11 +279,13 @@ class PageController extends Controller
         $page = $this->pageRepo->getById($pageId);
         $this->checkOwnablePermission('page-update', $page);
         $this->setPageTitle(trans('entities.pages_delete_draft_named', ['pageName' => $page->getShortName()]));
+        $usedAsTemplate = Book::query()->where('default_template', '=', $page->id)->count() > 0;
 
         return view('pages.delete', [
             'book'    => $page->book,
             'page'    => $page,
             'current' => $page,
+            'usedAsTemplate' => $usedAsTemplate,
         ]);
     }