preview without upload

the toolset was having an issue retrieving large media files; e.g., > 5mb,
during “Step 3: Batch preview” of the batch upload process; it would return
a blank page or an error and the background jobs would not run properly or
not run at all.

i re-factored “Step 3: Batch preview” so that it no longer retrieves
mediafiles from the remote server, but instead presents the user
with a template preview of the first 3 items. once the user accepts
the preview results, the background jobs do the work of
retrieving the mediafiles.

Bug: 63864
Change-Id: I91fa14730b9614cef4b912b218a692fb19271ce9