This was extracted from AfterSigning method and is needed when
we add the certificate chain to the database after the document
was loaded already.

Change-Id: I3087386d0131ce962eb3b588c409542617eb7bfe
Reviewed-on: https://gerrit.libreoffice.org/62275
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>