Autosave in the background

[Macroz]

Autosave in the background by replacing/patching the latest draft-saved event. If the latest event is not a draft-saved then create a new one. This may require some event handling logic to change.

See https://github.com/CSCfi/rems/blob/master/docs/architecture/017-applicant-ux.md.

• automatic saving in the background
• compacting consecutive saves into one

[Macroz]

The feature is now ready to experiment with and test. A few leftover ideas and tasks for changing into default have been recorded to #3037.