Improve CertificateManager to not be user context dependent

* removes the ability for users to import their own certificates (for external storage)
* reliably returns the same certificate bundles system wide (and not depending on the user context and available sessions)

The user specific certificates were broken in some cases anyways, as they are only loaded if the specific user is logged in and thus causing unexpected behavior for background jobs and other non-user triggered code paths.

Signed-off-by: Morris Jobke <hey@morrisjobke.de>