Payment subtotal amount fixed

Function edd_get_payment_subtotal() should return the amount without any
discounts applied, that means it can not take total amount and just
subtract the taxes, like it was doing.
Allow empty fields to take into account custom template

If for some reason a custom shortcode template doesn't use display name, first/last name or email address it results in a undefined index. Ideally these should be optional and if they aren't provided they should fall back to the original values.
Merge pull request #2523 from elliott-stocks/patch-5

Allow empty fields to take into account custom template
Sanitize amounts to PayPal
Merge pull request #2379 from somewebmedia/fix/payment

Payment subtotal amount fixed
Check that ['subtotal'] isset to protect against notices with really old payment records. #2379
Fix merge conflict