[feature] add edition suggestion

[jibe-b]

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

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