Only show case type select if creating new cases
Do not allow blank external ID's

This prevents users from uploading files matched on external id's with
blank externals. Not a breaking bug but it caused some pretty weird
results.
Hide some of importer success rows given 0 counts
Fix issues with date conversion

Date conversion errors would cause the import to hang. Now it finishes
and displays a message informing the user of the failed row.
Merge pull request #562 from dimagi/importer-bugs

Case importer fixes/changes