Improved notices in the add_to_cart() method

- Moved the button markup out of the translation strings.
- Used get_cart_url() instead of get_permalink( wc_get_page_id( 'cart' ) ).
- Added some newlines to improve code readability.
Merge pull request #4453 from GeertDD/add_to_cart_notices

Improved notices in the add_to_cart() method