add support for module items in selective content export

test plan:
* should be able to use the API to export individual items
associated with modules by specifying a "module_item" or
"content_tag" id

refs #CNVS-14552

Change-Id: I340c5e35297aa9d6e7971ba2b26ae6b222404fde
Reviewed-on: https://gerrit.instructure.com/39808
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>