Clear secreturl access cookies on logout

Bug 1385564: This doesn't provide much additional security, because if
the access cookies are still in your browser session, then the secret URL
itself is probably still in your browser history. But if someone goes to
the trouble of logging out *and* clearing their browser history, this
will ensure that it actually does end the secreturl access cookie like
they'd expect.

Change-Id: Ia75f58015ab2cb54c9184cdc8b5bf32dfe543733