Real support for sub pipelines and improve UI for it

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

Currently you can run "sub pipelines" but it's only by proxy of triggering pipelines with the API. But we don't have true sub-pipeline support.

The UI needs to improve to show that we actually support this.

The work is already done but I believe a UI just needs to be put in place.