Add a default tax address preference

[mamhoff]

The current default_tax_zone system is flawed: It conflates tax and
shipping zones (Spree::Order#tax_zone can return a shipping zone if that
happens to be smaller than the smalles matching tax zone), and it creates
a large number of complex queries that won't be necessary once we start
finding tax rates by the order's tax address.

However, the default tax zone was good for one thing: Tell the system which
taxes to use for orders in cart state (before any addresses are entered).

This commit introduces a default tax address preference, defaulting to an
address with the default country as country. The object is also frozen so
the taxation system doesn't go on building many useless objects.

This PR should also ease @jhawthorns concerns about using the same preference for the default country being used for taxation purposes: If you want to override it, specify a different default tax address.

[mamhoff]

Superseded by #862