CSRF tokens expire and break the web UI

If a user has the browser open for more than a day the CSRF token expires and then the UI breaks in mysterious ways - most of the GET methods appear to work ok but POST methods raise Unknown Error. So user will navigate as normal around the UI but when they try to launch a flow it will break with an unknown error.

Its not a big deal but users get the impression that GRR is flaky and its a bad user experience. A full refresh fixes the issue so maybe the GUI should force a refresh if it detects this condition.