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/Models/Book.php

@@ -15,6 +15,7 @@ use Illuminate\Support\Collection;
  *
  * @property string                                   $description
  * @property int                                      $image_id
+ * @property ?int                                     $default_template
  * @property Image|null                               $cover
  * @property \Illuminate\Database\Eloquent\Collection $chapters
  * @property \Illuminate\Database\Eloquent\Collection $pages