[FIX] mail: keep ratio of img in message with inline dimensions

Before this commit, when a message body contains a big image with
inline dimensions, the image was stretched.

For instance:

- `<img width="4096" height="2304">`
- on 1080p screen in the discuss app:
- max-width for image: 1542px
- image ratio: ~ 1,78
- expected height: 867.38px
- actual height: 2304px

This issue had been fixed[1], but was reintroduced[2].

Note that this fix only works when inline dimensions are set as attr
of the `<img>` tag. It won't work with dimensions attached as inline
styles (e.g. `<img style="height: 4096px; width: 2304px;>`), due to
high CSS specificity of inline styles.

[1] https://github.com/odoo/odoo/commit/e7bc4e9703954fa952f24b4f96e75dc4299bbb28
[2] https://github.com/odoo/odoo/pull/15542/commits/46699c5214229ea8aebcc04e08d0786fd2b67f35

Closes #36227

closes odoo/odoo#36243

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/less/thread.less

@@ -135,6 +135,7 @@
             }
             img {
                 max-width: 100%;
+                height: auto;
             }
 
             .o_mail_body_long {