Original PR description

Impacted modules: portal, rating, website_rating, website_slides

This merge modifies the some aspect of posting a message for public 
document, in order to implement the rating (message+rating) on a slides 
channel.

We now support the use case of posting without having the right to write 
on a document, and force the author of the message (as it is required to 
be logged to post a message). The security has been improved: to do so, 
the document token is not leaked anymore. Only the signed token (by current 
partner id) is used.
The rating mixin has been modified to handle displaying average and count 
(per document), in an efficient way.

A new frontend message composer is born in website_rating (as it allow to 
post a rating with a comment). To do so, a little refactoring of the 
Portal Chatter has been done.

For more details, please have a look at the subcommits. They are well
explained and correctly split.

Task-1902304