[FIX] mail: disable the sidebar when opening a pdf in the chatter

The pdf viewer sometimes displays the sidebar when we open the pdf

Solution:
Add `pagemode=none` to the url when opening a pdf in attachments

opw-3193516

closes odoo/odoo#120347

X-original-commit: da9b89ad
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>
Signed-off-by: Guillaume Merlin (megu) <megu@odoo.com>