Before this PR cache control header for avatar was set to no-cache which is not the right value. This PR mark the avatar routes response as immutable when a unique string is present. We also use the partner write_date as unique string when possible. task-3551636 closes odoo/odoo#141326 X-original-commit: 3de4f1f1225229cbdb0512df5790ce491f04ea7e Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>