Implement time travel (undo/redo/revert) of auto-saved states across devices and users

Another thing to add here is that if we do decide to implicitly save a revision every so often when an autosave state is valid/ready to save, then it would be possible to revert to those interim revisions instead of the auto-save state itself. This does not help with a full collaborative editing workflow but might be an interim step in lieu of one which would avoid having to revert to actual autosave states as such.

Re #4, if we want to pursue an interim solution where we either retain a partial history of auto-saved non-revisions, or a partial history of auto-saved actual-revisions, I'd recommend we do it as logarithmically-spaced snapshots. That way, even if the content author makes 1000 autosaved edits prior to sending their work into a "for review" state, we'd only need to save/retain 10 or so states/revisions, which actually seems quite manageable even without any of the libraries proposed in this issue. https://madebyevan.com/algos/log-spaced-snapshots/ has a clever algorithm for how to do this in a way that only requires at most one deletion per save. That post shows an example with a "density" of 2, but we might find that even a density of 1 is sufficient.