Adapt `restoreFiles` to collections

[jgaehring]

I was in the middle of adapting the restoreFiles composable so that it would work for relationships to collections of file entities, not just single file entities, when I had to put all of this down. Most file entity relationships will be collections, so this has to be resolved before any other issues related to images and the Observations module.

Once that's done, there hopefully shouldn't be any other major work to do with the logic for syncing and caching entities in FK core and I can complete the main work for the Observations module of the creating a basic UI (#507).