CLC-4009 Client-side fix for various LTI-masquerade oddities

https://jira.ets.berkeley.edu/jira/browse/CLC-4009

Also fixes a long-time misleading description of the original-user-ID in the footer.
This is necessary because authenticating via LTI may trigger a "Can't verify CSRF token authenticity" error later. But when the session is authenticated only by LTI, a redirect to the settings page is all that's needed to clear up the state.