Make "Remove" from Cart Button work within Ajax calls

In the "edd_remove_item_url" function, there is a Boolean parameter for $ajax - however, it isn't actually used - neither is it actually necessary.

I have modified the function so it can be used in ajax functions and uses the checkout page by default.

The change involves checking the "DOING_AJAX" constant, and if we are using ajax, we use the edd_get_checkout_uri() function as the $current_page used to create the URL in the "Remove" button.
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Add current_user_can and better escaping to edd_check_for_download_price_variations()
Check post type in edd_check_form_download_price_variation()
Improved formatting of product details widget
Improved escaping for EDD_Product_Details widget
Improved escaping for widgets
Check that current user can manage shop settings for license activation and deactivation
More unique key for payment key
Use AUTH_KEY in payment key generation
Introduce generate_public_key(), generate_private_key(), get_token() and improved API key generation
Cleanup updater class
Fix issue with updater running even when license key is inactive
Don't allow cart quantity to be set less than 0 via ajax. Closes #2213
Fix for when there is no $post on the page

This gives support for Pods Pages which can be created as standalone URLs apart from Post archives and singular templates. In certain cases, $post is not setup (like a 404 would have) and this throws errors when trying to utilize edd_get_purchase_link and passing in $args to manually set download_id.
Merge pull request #2215 from sc0ttkclark/patch-1

Fix for when there is no $post on the page
Add edd_is_success_page()
Merge pull request #2221 from leewillis77/is_success_page

Add edd_is_success_page()