Sync and cache images and other files.

[jgaehring]

As a preliminary to alpha.9, this establishes the internal mechanisms for syncing and caching files, which is the bulk of that work required for the Observations module. As I mentioned on the forum, I'm tempted to merge this work into main just to confirm that everything works on https://develop.farmos.app, and these commits do in fact represent a relatively complete feature set for FK Core, even if the Observations module itself is far from complete (not a ton of work remaining there, but there just isn't much of a UI to speak of).

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