Fix 'Invalid nonce' error on EDD edit discount code link

[zackkatz]

Summary

• The "edit discount code" link on the Sitewide Sale edit page used edd-action=edit_discount, which triggers EDD's discount save handler and requires nonce validation
• The correct URL parameter for viewing the edit screen is view=edit_discount, which simply loads the editor without requiring a nonce
• Fixed in both the PHP template (server-rendered link) and the JavaScript (dynamically-updated link when changing the dropdown)

Fixes #183

Test plan

• Create or edit a Sitewide Sale with EDD sale type
• Select an existing EDD discount code
• Click the "edit discount code" link — should open EDD discount edit page
• Change the selected discount code in the dropdown, then click "edit discount code" — the dynamically-updated link should also work