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.

database/migrations/2023_12_02_104541_add_default_template_to_books.php

@@ -14,7 +14,7 @@ class AddDefaultTemplateToBooks extends Migration
     public function up()
     {
         Schema::table('books', function (Blueprint $table) {
-            $table->integer('default_template')->nullable();
+            $table->integer('default_template')->nullable()->default(null);
         });
     }