Case importer fixes/changes

These commits address feedback from Sheel's QA.

#### Only show case type select if creating new cases

Move case type selector below the "create new cases" link and hide it if not checked to make it more clear this is only for new cases, not updating.

#### Do not allow blank external ID's

Prevent the user from leaving external ID blank is they are matching cases based on this. Behavior was pretty weird otherwise and an empty string for an ID doesn't seem correct.

#### Hide some of importer success rows given 0 counts

It was showing stuff like "0 rows had duplicate matches found..." etc which didn't really make 

#### 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.