Improve display of SSVC in UI

[pombredanne]

I would like to have a better display of SSVC trees in the UI. Either use something like YANML rather than a Python repr or JSON, or as a visual tree

This is a follow up to this PR and issue:

• Collect SSVC trees #2050
• cravex2-reachability: Collect available SSVC in VulnerableCode #1963