Move notice templates into /notices/notice.php
Store all notices in 'wc_notices' array

With 'error', 'success' & 'notice' array keys
Introduce new wc_get_notices() function

To replace the deprecated Woocommerce->get_messages() and
Woocommerce->get_errors() functions
Introduce new wc_print_notice() function
Use wc_print_notice() for checkout notices
fixup! Store all notices in 'wc_notices' array
Use $messages variable for all notice templates
Use new notice template for Shipping Updated

For #4034