[FIX] mail: thread: keep image ratio

With rule 'max-width: 100%', the height must be reset to 'auto' to keep the
image ratio.

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

@@ -131,6 +131,7 @@
             }
             img {
                 max-width: 100%;
+                height: auto;
             }
 
             .o_mail_body_long {