Adapt URL to DLA advisories in a https://www.debian.org/security/lts/

As discussed in https://bugs.debian.org/859122 DLAs and DSAs will be
separated in different supages. This needs adaption for the URL
referenced in the source fields of the security-tracker for DLAs.

v2: Correct URL to actually match the final location under
    /lts/security. Cf. https://bugs.debian.org/859122#82

Thanks: Laura Arjona Reina, Holger Levsen and Antoine Beaupré