Use issuer SKI for AKI when transferring CSR extensions.

[jonwltn]

This PR fixes a problem where KSE will use the wrong authority key identifier when transferring extensions during CSR signing.

GnuTLS certtool uses the SHA-1 hash over the SPKI for the AKI and SKI extensions. This deviates from RFC 5280 method 1, which is the default method used by KSE.

KSE was updated to use the issuer SKI when applying the standard extension template in Add Certificates (see #560), but KSE continued to use RFC 5280 when transferring the CSR extensions to the newly signed certificate. If a user chooses to transfer the extensions rather than applying the standard template when signing the CSR, the cert chain will be invalid.

After this PR is merged, KSE will now use the issuer SKI for the issued certificate's AKI when:

• Applying standard template (Custom Authority and Subject Key Identifiers #560)
• Applying a saved template (this PR)
• Transferring extensions from a CSR (this PR)

[kaikramer]

Thanks!