Additionally logout from a settings list of extra logout URIs

Currently, the LMS logout endpoint should iframe in the logout pages of
all the IDAs you were logged into. In short, this was made possible with
DOP because keeping track of the logout URIs and leaving a trail of
evidence in the user cookies was part of what we added in our fork of
DOP.  In the case of DOT, we don't have time or desire to fork DOT to
mirror this behavior, so our stop-gap solution is to log out the user
from a list of logout URIs in settings.