Got book to shelf conversions working

- Also extracted shelf to book view elements to own partial.
- Fixed some existing logic including image param handling in update
  request and activity logging against correct element.

app/Http/Controllers/BookshelfController.php

@@ -167,7 +167,7 @@ class BookshelfController extends Controller
 
         if ($request->has('image_reset')) {
             $validated['image'] = null;
-        } else if (is_null($validated['image'])) {
+        } else if (array_key_exists('image', $validated) && is_null($validated['image'])) {
             unset($validated['image']);
         }