Handle advisories which have related/same data

[TG1999]

We have advisories which are almost identical, also some kind of advisories which report different data but share same aliases and identifiers. And also sometime same advisory from the upstream gets updated. We need to handle all these cases.

• Same unique content ID, different AVIDs (same advisory) - We need to have a relationship to basically determine and assign primary and secondary advisory accordingly. We can decide it at creation time or have an improver
• Different content ID, shared aliases and identifiers: It could still be the same; what we need is specific improvers to know if it's a new or existing advisory. We can relate to these kind of advisories.

Beyond....

• Different content ID, same AVID (update): This is an update to the same advisory, which is processed using Plan storing of multiple advisories with same AVID #2068