Make BulkUpload::Attachments just BulkUpload

Turns out it makes a lot of sense to requisition the BulkUpload class
for our purposes here, and make it an ActiveRecord-like class that we
can use for the intermediary step when we process uploaded zip files.
WIP on bulk upload process

We now have a separate controller for the management of bulk uploads.
It will utilise our ActiveRecord-like object to eventually save a
collection of Attachments to an edition.