Lowest and highest price options

This allows price keys not to be ordered like [0,1,2...] but also can be
random ids if needed
Set same meta on cart items added manually to a payment, as when normally purchased
Round item price before storing
Add filter for credit card images
Fix spacing
Change filter name
Merge pull request #2406 from leewillis77/issue_2404

Set same meta on cart items added manually to a payment
Add error for discount duplication
Helps to add all relevant files...
Fix download log lookup by email/id
Add filters to add/edit discount pages
Add post and body classes
Merge pull request #2415 from ghost1227/issue/2414

Add error for discount duplication
Tweak error message for #2414
Tweak return value names for consistency
Minor tweaks
Properly handle item quantities when adding to cart
Add is_singular check and class
Merge pull request #2422 from ghost1227/issue/2391

Add filters to add/edit discount pages
Sanitize text return