Added check to ensure apply button is clicked after entering a discount code

A js check has been added to ensure that a coupon is applied using apply button before proceeding to the next page so that the amount processed by stripe extension and CiviCRM remains the same. This will require https://github.com/civicrm/org.civicrm.module.cividiscount/pull/263 to work as it uses the discount-applied attribute to check if the discount has been applied or not.