[Task] Rework volume "import" process to allow incremental document load

[mradamcox]

Describe the task
Background on this idea is that often in big cities, people are only specifically interested in certain neighborhoods. This is tricky because currently an entire volume must be loaded into the system (copying all of its image files to the server) before work can begin on any specific sheet. It would be better to break this down a bit...

Approach
One way to handle this, would be to change the workflow such that:

1. "Loading" a volume creates a new Volume instance and descendant Sheet instances.
2. These Sheets do not have Documents associated with downloaded files, they just have a link to the IIIF resource for each sheet.
3. The Unprepared section of the Volume Summary page shows this same tiles as now, but these are just thumbnails from the IIIF server (perhaps best saved locally?)
4. "Preparing" a sheet would queue the actual download of the jp2 from the remote server. Exactly when/how that would take place is TBD. Could the entire preparation interface be powered from the IIIF endpoint, not a local file? Most importantly, would the coordinates/dimensions translate properly?

A benefit of this approach would be a move toward better integration with IIIF services.