Rolled out reference pages to all entities, added testing

Including testing to check permissions applied to listed references.

app/Http/Controllers/ReferenceController.php

@@ -3,7 +3,12 @@
 namespace BookStack\Http\Controllers;
 
 use BookStack\Auth\Permissions\PermissionApplicator;
+use BookStack\Entities\Models\Book;
+use BookStack\Entities\Models\Bookshelf;
+use BookStack\Entities\Models\Chapter;
+use BookStack\Entities\Models\Entity;
 use BookStack\Entities\Models\Page;
+use Illuminate\Database\Eloquent\Collection;
 use Illuminate\Database\Eloquent\Relations\Relation;
 
 class ReferenceController extends Controller
@@ -23,8 +28,64 @@ class ReferenceController extends Controller
     {
         /** @var Page $page */
         $page = Page::visible()->whereSlugs($bookSlug, $pageSlug)->firstOrFail();
+        $references = $this->getEntityReferences($page);
 
-        $baseQuery = $page->referencesTo()
+        return view('pages.references', [
+            'page' => $page,
+            'references' => $references,
+        ]);
+    }
+
+    /**
+     * Display the references to a given chapter.
+     */
+    public function chapter(string $bookSlug, string $chapterSlug)
+    {
+        /** @var Chapter $chapter */
+        $chapter = Chapter::visible()->whereSlugs($bookSlug, $chapterSlug)->firstOrFail();
+        $references = $this->getEntityReferences($chapter);
+
+        return view('chapters.references', [
+            'chapter' => $chapter,
+            'references' => $references,
+        ]);
+    }
+
+    /**
+     * Display the references to a given book.
+     */
+    public function book(string $slug)
+    {
+        $book = Book::visible()->where('slug', '=', $slug)->firstOrFail();
+        $references = $this->getEntityReferences($book);
+
+        return view('books.references', [
+            'book' => $book,
+            'references' => $references,
+        ]);
+    }
+
+    /**
+     * Display the references to a given shelf.
+     */
+    public function shelf(string $slug)
+    {
+        $shelf = Bookshelf::visible()->where('slug', '=', $slug)->firstOrFail();
+        $references = $this->getEntityReferences($shelf);
+
+        return view('shelves.references', [
+            'shelf' => $shelf,
+            'references' => $references,
+        ]);
+    }
+
+    /**
+     * Query the references for the given entities.
+     * Loads the commonly required relations while taking permissions into account.
+     */
+    protected function getEntityReferences(Entity $entity): Collection
+    {
+        $baseQuery = $entity->referencesTo()
             ->where('from_type', '=', (new Page())->getMorphClass())
             ->with([
                 'from' => fn(Relation $query) => $query->select(Page::$listAttributes),
@@ -39,9 +100,6 @@ class ReferenceController extends Controller
             'from_type'
         )->get();
 
-        return view('pages.references', [
-            'page' => $page,
-            'references' => $references,
-        ]);
+        return $references;
     }
 }