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.
Add edd_is_success_page()
Merge pull request #2221 from leewillis77/is_success_page

Add edd_is_success_page()
Merge pull request #2171 from moveplugins/master

Make "Remove" from Cart Button work within Ajax calls
Fix hanging else, props @markjaquith
Fix space in filter name
Add $payment_id to download title filter

There may be info within the payment meta to add to the title. Passing the $payment_id makes this possible.
Merge pull request #2242 from elliott-stocks/patch-16

Add $payment_id to download title filter
Use a non-broken cache key
Merge pull request #2243 from leewillis77/stats_fix

Use a non-broken cache key
Fixed rendering of text elements

- Fixed bug in EDD_HTML_Elements::text(), which caused the rendering of broken HTML.
Merge pull request #2250 from daigo75/master

Fixed rendering of text elements
Add class to download log

Make it easier to target for hiding if selling physical products
Add trigger when quantity is updated

Useful for changing the style of the cart count
Merge pull request #2252 from elliott-stocks/patch-19

Add trigger when quantity is updated
Merge pull request #2251 from elliott-stocks/patch-18

Add class to download log