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/Search/SearchController.php

@@ -2,6 +2,7 @@
 
 namespace BookStack\Search;
 
+use BookStack\Entities\Models\Page;
 use BookStack\Entities\Queries\Popular;
 use BookStack\Entities\Tools\SiblingFetcher;
 use BookStack\Http\Controller;
@@ -82,6 +83,32 @@ class SearchController extends Controller
         return view('search.parts.entity-selector-list', ['entities' => $entities, 'permission' => $permission]);
     }
 
+    /**
+     * Search for a list of templates to choose from.
+     */
+    public function templatesForSelector(Request $request)
+    {
+        $searchTerm = $request->get('term', false);
+
+        if ($searchTerm !== false) {
+            $searchOptions = SearchOptions::fromString($searchTerm);
+            $searchOptions->setFilter('is_template');
+            $entities = $this->searchRunner->searchEntities($searchOptions, 'page', 1, 20)['results'];
+        } else {
+            $entities = Page::visible()
+                ->where('template', '=', true)
+                ->where('draft', '=', false)
+                ->orderBy('updated_at', 'desc')
+                ->take(20)
+                ->get(Page::$listAttributes);
+        }
+
+        return view('search.parts.entity-selector-list', [
+            'entities' => $entities,
+            'permission' => 'view'
+        ]);
+    }
+
     /**
      * Search for a list of entities and return a partial HTML response of matching entities
      * to be used as a result preview suggestion list for global system searches.