revised wc_get_product_terms to allow override with args
[2.1] Fix coupon limit checks and enhance to check ID by provided email (if logged out)

Fixes #5535
fixing danish krone symbol
Check for Existence of Cart Before Using

check for the existence of the cart during the is_available() check
before getting the order total (which otherwise will result in a PHP
notice from the admin, etc)
Fixes performance degredation on large wp_options tables
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