Make case type meaningful on import

These commits address some issues that came up during importer QA. Namely, case type wasn't being used properly.

1. We hid the case type for update only imports. This was reverted.
2. We never updated cases with the wrong type selected but we would tell the user that we did. Now we don't count those.
3. External ID updating in case updates was actually broken, oops.
4. External ID will only match on the specified case type, no longer requiring the external id to be unique across all apps.