Move importapiusers mgt command to api app

I had originally put the `importapiusers` management command in the
`sierra` app in order to keep it with the other commands, but this one
actually needs to be run in production, and the `sierra` app is not
installed there. It makes more sense to keep it with the `api` app, so
I've moved it there.

Author: jthomale

django/sierra/api/management/__init__.py

@@ -9,7 +9,7 @@
 from django.core.management.base import CommandError
 from django.utils.six import StringIO
 
-from sierra.management.commands import importapiusers
+from api.management.commands import importapiusers
 
 
 # FIXTURES AND TEST DATA