[MERGE] website_{rating, slides}: Manage attachments in review update

PURPOSE

Allow to manage attachments when updating a frontend review.

SPECIFICATIONS

Currently when posting a review in eLearning or eCommerce one can attach
files to its review thanks for portal composer. Rating popup composer allow
to update its comment and rating. However this edit mode has two limitations.
It does not display attachments previously added and does not allow to add
attachments.

This merge fixes that by

* giving attachment values to the composer, allowing its display by the
composer display at startup;
* allowing to give new attachments and properly handle them in slides
route;

Uploaded and validated attachments cannot be removed by users. Indeed
message with attachments has probably already be sent to people and removing
attachments would lead to information loss.

LINKS

Task 2066600 (manage attachments)
Task 2058595 (eLearning v13 testing)

closes odoo/odoo#36409

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>