Ability to control whether new sorting-room mints a new canvas id

[tomcrane]

At the moment sorty mints a new canvasId (in fact clones the canvas from the source manifest and gives it a new ID, then uses it in the derived manifest). Here's that in the original POC:

https://github.com/dlcs/sorting-room/blob/proof-of-concept/script/sorty.js#L78

We need to be able to control whether sorty does this or not. This is probably a config option for a particular deployment of sorty, that can be overridden by the user (but careful to avoid doing this unintentionally). For IDA, once we have the source rolls as already-processed manifests in Presley, we don't want to re-mint the canvas IDs.

If it doesn't do it, it simply pushes the existing canvases to the new manifest.

@andyvidual - is there a way to do this easily?