The end user should be informed when the form has been saved

## Description
We have now implemented autosave on onBlur. There is no indication to the end-user that the form has actually been saved. My hunch is that this is going to be confusing for the end-user, and we should have a way of informing the user that the form has been saved.

This issue also needs to handle how we will show the user that auto-save has failed in some way. This was discussed in Altinn/altinn-studio#3826

## Considerations
Is this really a need?
- Seems like it, theres been doubts that the exit button is "safe". The problem is that the user wont know if the form is saved or not. This could be fixed by somekind of message telling the user that the form has been autosave or a "fake" save and quit button.
- User tests show some users are unsure if their work will be saved.

## Acceptance criteria
- The user is informed/knows that the form has been saved when they make changes

## Specification tasks

- [ ] Design: How/where to show a "saving state" somewhere in the app GUI
- [ ] Take inspiration from MS Office?
- [ ] Always near the X? Or a hovering component always shown in the same place?
- [ ] Tech: How to ensure that "saving state" is shown correctly (or at least sufficiently correct...)
- [ ] Mirroring the Redux state?
- [ ] Test design / decide test need

## Development tasks
> Add tasks here

## Definition of done
Verify that this issue meets [DoD](https://confluence.brreg.no/display/T3KP/Definition+of+Done#DefinitionofDone-DoD%E2%80%93utvikling) (Only for project members) before closing.

- [ ] Documentation is updated (if relevant)
- [ ] Technical documentation (docs.altinn.studio)
- [ ] User documentation (altinn.github.io/docs)
- [ ] QA
- [ ] Manual test is complete (if relevant)
- [ ] Automated test is implemented (if relevant)
- [ ] All tasks in this userstory are closed (i.e. remaining tasks are moved to other user stories or marked obsolete)

Start by reviewing the autosave-on-blur behavior and the related discussion in Altinn/altinn-studio#3826. Then examine how the saving state is represented in Redux and surfaced in the app GUI. Done means the design, failure behavior, testing needs, and user-facing saved indication are agreed and implemented.

Written by the indexing model from the issue text.