[feature] add edition suggestion

Edition suggestion _i.e._ can already be handled in a _ad hoc_ way using h like:
- edit an annotation
- indicate that this is an edition suggestion
- the author of the document reads the annotation
- modify according to this suggestion

This process could be simplified, at least on the side of h by:
- extend the annotation model to describe a suggestion
- add an "edition suggestion" mode to the UI, which fills the "edition suggestion" key by default

Then, the webpage author should have an inbox/a way to receive a notification of such a suggestion.

This feature could then enhance etherpad [issue](https://github.com/ether/etherpad-lite/issues/3277)

The inbox could be handled by the LinkedDataNotification recommendation cc @csarven