all-js add/edit volume

[boyska]

We have a plan for a brand new "add a volume" page. First of all, it should be as similar as possible to the "edit volume" page.

It should be basically divided into two "forms": metadata and attachments. This reflects archivant structure, and allows a user to upload a file and, while the transfer is going on, she can fill the metadata.

This would also require no additional API methods. The only "strange" part is: what should we do if a user wants to start uploading a file before having created the volume? We decided that the best possible thing is just writing some simple JS that, in that case, creates an empty volume with no metadata (completely ok) and attaches the file to it. Of course the user isn't supposed to upload a file and set no metadata at all, but from archivant point of view, it's admitted.