Disambiguate redux actions which modify notebook content / title due to a user action and ones that don't

Per the discussion here https://github.com/iodide-project/iodide/pull/1812#discussion_r284920334, we'd like to somehow distinguish between notebook content/title changes which happen due to the user doing something (i.e. modifying the notebook) and ones that happen due to e.g. reverting to a previous revision from the revision browser. The first type should trigger an autosave, while the second shouldn't.

I would do this in #1812 but that PR is already quite big and I think this would be easier to do/review as a followup.

Start with PR #1812 and the linked discussion to understand the existing Redux actions and autosave behavior. Trace how user edits and revision-browser reverts modify notebook content or title. Done means those actions are distinguishable so user changes trigger autosave while revision reverts do not.

Written by the indexing model from the issue text.