UI: Payment Page clean-up

[julianajlk]

Clean-up include:

• Post and Delete coupon now return the complete basket page (bug on saga with duplicate addCouponSuccess is fixed).
• Discount line no longer displayed when there is no discount .
• "In your cart" to "In Your Cart"
• Changed section headings to h5 className.

FYI @edx/arch-squad

[codecov]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #47   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          31     31           
  Lines         317    319    +2     
  Branches       44     45    +1     
=====================================
+ Hits          317    319    +2

Impacted Files	Coverage Δ
src/payment/ProductLineItem.jsx	100% <ø> (ø)	⬆️
src/payment/ProductLineItems.jsx	100% <ø> (ø)	⬆️
src/payment/PaymentForm.jsx	100% <ø> (ø)	⬆️
src/payment/coupon/data/service.js	100% <100%> (ø)	⬆️
src/payment/BasketSummary.jsx	100% <100%> (ø)	⬆️
src/payment/coupon/data/sagas.js	100% <100%> (ø)	⬆️

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