[FEAT] Reduce Code Complexity of Demo's Checkout Module

[xelaint]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Other... Please describe:

Current behavior

The demo's cart module has a two extremely complex components:

1. checkout-view
2. cart-summary-wrapper

that are really hard to understand. They interact with store and also have @Input's.

3. demo-cart-summary-wrapper tests in checkout-view are xdescribed because they are hard to understand. Tests should be more specific to the wrapper rather than using first, second, third, etc.

Expected behavior

There are a few more smaller "dumb" components" and a potentially a few more "smart" components to lower the overall complexity of the module.

What is the motivation / use case for changing the behavior?

Maintainability and code legibility.