Disable Google Checkout for gilding via purchase-gold page.

Follow up to commit 399f0c6 which intended to disable Google Checkout for gilding comments.

The above commit succeeded for the gold form that is injected into the DOM when a user clicks the 'give gold' button, however if the user follows the link's href to the purchase-gold page (`https://ssl.reddit.com/gold?goldtype=gift&months=1&comment=<comment ref>`) the Google Checkout button is rendered.  This pull request corrects that.