[FIX] web_editor: missing record info for converted images

When the Wysiwyg is used by website, multiple editable elements are
possible, and each of them carry the related record's model and id in
the element's dataset.

On the other hand, when the Wysiwyg is used by the html field, there's
only one root editable element, and such information is not present in
the editable's dataset, but in the widget's options object.

Before this commit, when saving the content of an html field, modified
images were converted to attachments without the correct reference to
the record's model and id.

This commit makes sure that such information is passed to the route that
handles attachment creation.

task-3495668

closes odoo/odoo#134705

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>