Add overwrite dialog for SAVE_FILES case

[mcclurgm]

Problem

If there are conflicts of the URIs to be saved by a SAVE_FILES portal request, we don't prompt the user for confirmation and instead silently overwrite their data.

Proposal

See #1989: follow that implementation, but for the multiple files case.

Prior Art (Optional)

I'm not aware of how other portal implementers or systems do this, but if I find any examples I'll edit to add.