The new "country access" implementation replaces the old
implementation.  Middleware and tests have been updated
accordingly, but deprecated models are preserved
for backwards compatibility.

Add Django admin UI for configuring country access

Migrate existing embargo rules into the new tables.

Finalize tests for embargo middleware app

Allows specification of countries to embargo, what course(s) should
apply embargo restrictions, and whitelist/blacklist capability for
specific individual IP addresses.