Dependency Track integration: Remove environments on deletion

[bomoko]

Currently, the dependency track integration only captures scans. The problem is that, especially for ephemeral environments, deleted environments potentially leave the DT instance littered with outdated data about environments long gone.

We need to be able to, on namespace deletion, also delete the corresponding DT environment (assuming it exists).

[seanhamlin]

Also, when a project is deleted, delete the root project in DT as well.

[bomoko]

@seanhamlin - that's slightly more complicated, given that remote doesn't necessarily know when a project is removed. This, then, becomes something that needs to happen on the API side.

[bomoko]

I suppose that what I could do is just remove the parent project if it's empty. That might get race-y, potentially. I'll give this some thought.