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.
Update abstract-wc-email.php

Adds a plain text replacement for the dollar sign encoded as html entity.
Add delete tax rates option to system tools
Don't resort meta in admin
wc_has_notice function
Merge pull request #4458 from ChromeOrange/master

Add delete tax rates option to system tools
Merge pull request #4457 from PURPORC/patch-1

Update abstract-wc-email.php
Merge pull request #4453 from GeertDD/add_to_cart_notices

Improved notices in the add_to_cart() method