CookieJar refactoring required to overcome issues in Shibboleth support

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.