Revamp the favorites screen

# Description
Right now the screen is completely blank whenever there are no favorites, and still looks empty when a user has favorite books.

# Relevant files
- `Favourites.dart`

# Screenshot
<img src="https://user-images.githubusercontent.com/74055102/147944883-7fa80336-9f45-4229-b9b9-a8b29688b834.png" height=500/>
