Remove bulk uploads from attachment_fields partial

Only editions use bulk uploads, and this partial is no longer used
with editions. Therefore we can drop all this logic from the partial.
Scenario for new bulk upload workflow
Start of an AR-like object for bulk uploads

This object represents a collection of attachments that are to be
saved to a "thing" (for now, probably just editions). We use this in
the bulk upload process, allowing the user to supply attachment titles
before we save them on-mass. Work in progress.
BulkUpload::ZipFile#extract_files returns paths

We no longer need separate filenames
Fix typo in form partial
Kill off old attachment management code

This commit removes all the code relating to the old way of managing
attachments. We no longer need to include the concern in our
controllers and we can kill off all the macros that tested the nested
attachment behaviour. We also kill off the code that was used to
manipulate the params based on any bulk uploaded zip files.

Functional testing of attachments will now happen in
Admin::GenericEditionsController::AttachmentsWorkflowTest. There are
a couple of test cases still to be added there.