refactor(checkout): simplify billing/checkout logic

[sethidden]

I refactored the Checkout and Billing pages a bit because it's the 2nd time something is returing from a retest and I was tired of the outdated logic in those components.

1. addressId are now always numbers (that's how they come from the api). String casts are used only when a function somewhere deeper down the line needs it
2. simplify logic of detecting whether the default address should be preselected. Preselection should only happen if a shipping/billing address weren't previously set on the cart. The logic from before PR doesn't quite work since the form approach changed since then
3. Make UserBillingAddresses use two way binding for selecting the address, rather than an internal computed. This re-introduces the behavior seen in M2-333 but that's intended
4. Show country name instead of country code in "billing address is the same as shipping" address card

M2-869