Add disk image transfer metadata form

This adds a new metadata input form which is provided when creating disk image transfers.

"Disk image" is exposed as a new transfer type, which operates identically to a standard transfer in every way when processing. When "disk image" is selected, a metadata edit button is added for each transfer component which allows a separate form to be filled out for each component. An edit button is also provided next to the "start" button, which lets the user pre-fill out a row to be attached to the next component that is added.

If a transfer metadata form exists for a given transfer, it will be associated with the "objects" directory of the final AIP produced from that transfer. This is accomplished by serializing it to disk, so it should work with SIP arrange as long as the arranged SIP contains contents from only one disk image transfer.

This adds a few forms to view the metadata form field values; there is code to edit them, but at the moment it's disabled by request.
Do these fields need to be defined in the form?  They're both in the model, and should be created automatically.  I think if a ForeignKey is not NULLable, then it is required in the form.  To set widget specifics for a field, use https://docs.djangoproject.com/en/dev/topics/forms/modelforms/#overriding-the-default-fields