The shibboleth implementation no longer maintains its own QNAM.
Instead, MirallAccessManager now holds a custom QNAM implementation
which saves cookies to a file on disk.
This patch also reduces some complexity wrt the browser window,
which used to be deleted via a roundtrip to its callee, which
is not longer required.
Fixes#1764 and Enterprise bug #165
Going forward, AbstractCredentials::getQNAM() could maybe removed entirely.
