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
closesodoo/odoo#141326
X-original-commit: 3de4f1f1225229cbdb0512df5790ce491f04ea7e
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>