Async star ratings

[jimchamp]

Closes #

Attaches on submit listeners to all star ratings forms on a page. On submit, the default page refresh is prevented and the ratings are submitted via JS.

If a submission fails, the rater is notified via toast message.

Technical

Replaced conditional rendering of "Clear my rating" button in template with conditional addition of hidden class, allowing the button's visibility to be toggled.
FadingToast constructor has been updated to prevent a null parent container being passed to the superclass Toast's constructor.

Testing

While logged in, do the following:

1. Navigate to a book page.
2. Leave a star rating. Ensure that the "Clear my rating" button appears, the correct stars are filled, and that the page does not refresh.
3. Refresh the page. Ensure that the rating is as expected.
4. Clear the rating. Ensure that filled stars are removed and that the button does not vanish.
5. Refresh the page and ensure that no stars are filled.
6. Navigate to your "Already read" log.
7. Leave ratings for multiple books on this page. When you submit a rating, ensure that the rating applies only to the intended book.

Screenshot

Stakeholders

[mekarpeles]

lgtm