(core) Support international addresses in the Billing form

Summary:
- When displaying, include the country code, and don't assume state is always present.
- When entering, include a country selector (defaulting to US), and
  make state/zip optional when non-US.
- Bring in an npm module with country codes.

Test Plan: Added a browser test case.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2647

package.json

@@ -64,6 +64,7 @@
     "fs-extra": "7.0.0",
     "grain-rpc": "0.1.6",
     "grainjs": "1.0.1",
+    "i18n-iso-countries": "6.1.0",
     "image-size": "0.6.3",
     "jquery": "2.2.1",
     "js-yaml": "3.12.0",