Bring back the automatic authentication popups

Bring back the automatic authentication popups

Users have complained that they don't see the notification when it is
shown and are not aware that their files aren't syncing.

Remove the non-interactive credentials fetch logic and add make sure
that the shibboleth popup will flash in the taskbar instead.
This will still not allow the popup to show in front in all cases,
but this is a compromise that we have to chose.

This reverts commit dcb687929f1323e9fea8a9e4bd48cdd3fa8312d9.
Issue https://github.com/owncloud/enterprise/issues/990