Disable tests when teachers logout & disable timeout for teacher/admin accounts

Fix for https://github.com/learningequality/ka-lite/issues/2403

We now disable exam mode automatically when a *teacher* (not an admin) logs out. This PR also disables the auto-logout due to timeout for teacher and admin accounts. 

Comes complete with tests @aronasorman !
ah, I see what you're saying -- this isn't actually where it would go, but I see what you mean about the decorator. I can put the check inside of the view function to simplify things