Add strict types for bookmarks management

Parameters typing and using strict types overall increase the codebase
quality by enforcing the a given parameter will have the expected type.

It also removes the need to unnecessary unit tests checking methods
behavior with invalid input.

application/front/controller/admin/ThumbnailsController.php

@@ -52,7 +52,7 @@ class ThumbnailsController extends ShaarliAdminController
         }
 
         try {
-            $bookmark = $this->container->bookmarkService->get($id);
+            $bookmark = $this->container->bookmarkService->get((int) $id);
         } catch (BookmarkNotFoundException $e) {
             return $response->withStatus(404);
         }