ARender: comments sync

Goal of the Epic is to handle the serialization of ARender comments using the comments service of Nuxeo Platform.
Expected outcome is:

• A better integration, from a user perspective, with other commenting input of apps using the platform
• Mutualisation of the effort of indexing generic coments in Nuxeo Platform for search (user searching on full text can find documents whose Nuxeo comments contains such a word)

As a custom project developer on top of Nuxeo Platform, I can use the Comments API (/ existing operations / Java Comments service API) to manipulate comments that are by default displayed in Arender.

Acceptance criteria

• If I add a discussion thread entry from Arender previewer, I can see the new comment thread entry in Nuxeo using the Nuxeo comment Service API.
  Date, author, and content of the comment are needed
• If I remove a discussion thread entry from Arender previewer on a given thread, and then use the Nuxeo Comment Service API to fetch the comment thread entries of the document, I don’t see the previously deleted discussion thread entry anymore. thus this deleted comment will not be seen anymore.
• if add a new comment thread entry using the Nuxeo Comment Service API on a comment thread related to an annotation linked to a document stored in Nuxeo, I can see this new comment from the Arender previewer in the corresponding conversation thread.
• if I remove a comment thread entry in a given comment thread bound to the document and corresponding to the discussion thread of an annotation, using the Nuxeo comment service API, I cannot see the discussion thread entry anymore from ARender Previewer when I load it on the document and display the corresponding Annotation discussion.
• if from the Arender Previewer I update a discussion thread entry in the discussion related to a given annotation of a given document, when I use the Nuxeo Comment Service API to fetch the comment threads related to the given annotation, the updated comment thread entry is indeed updated in the result of the API call.