Auto-submit coupon on press enter and avoid other form submit collision

Problem/Motivation

For the same reasons as per #2878155: When I press return to update the quantity of a line item it shouldn't be deleted, when user enters a coupon code and press enter, the first submit button of the form is triggered.
- If used from the checkout sidebar, it may go to next step
- When your coupon form is within the cart via #2872455: Allow coupons to be redeemed on the cart page, the first order item is remove from cart.

Even if not used within the cart, it makes sense that, if user press enter while writing a coupon code in the form, it submits the coupon.