Visualizing downstream pipelines in the upstream project (follow up to `triggered-by` MVC)

A number of items were out of scope for #9045 (closed) that will need to be addressed in subsequent releases. For the upstream projects, we're concerned about the UX in cases like:

• There are 100s of downstream projects that depend on an upstream one
• The upstream project is public but the downstream project is public
• What if both the downstream and upstream projects are public, but I only have access to the downstream one?