This PR is an attempt at refactoring the EditingRevision to make it match better with the way it's presented in the UI, as well as to simplify a lot of the processes in the backend.

Main changes:

• initial_state and final_state are replaced by a type. Each type corresponds to an editing action, either by the submitter, editor, or the system:
  • new - a submitter revision that hasn't been exposed to editors yet
  • ready_for_review - a submitter revision that can be reviewed by editors
  • needs_submitter_confirmation - an editor revision with changes the submitter needs to approve or reject
  • changes_acceptance - a submitter revision that accepts the changes made by the editor
  • changes_rejection - a submitter revision that rejects the changes made by the editor
  • needs_submitter_changes - an editor revision that requires the submitter to submit a new revision
  • acceptance - an editor revision that accepts the editable
  • rejection - an editor revision that rejects the editable
  • replacement - a system revision that replaces the current revision
  • reset - a system revision that resets the state of the editable to "ready for review"
• submitter and editor are replaced by just one user
• created_dt and reviewed_dt are replaced by just one created_dt
• Revisions no longer require having files. A revision now corresponds to an editing action, with or without files.
• New is_undone boolean column, which replaces the undone final state and the EditingRevisionComment .undone_judgment.
• All editing revisions (except for reset type) correspond to a top-level item/box in the editing timeline.