Improve the experience of starting migrations via the UI

We have several ways to approach this – it's possible that we'll want to build our own UI on top of the pipelineruns to be able to take them off of pending and get them started. Today it uses the native PipelineRun UI, and the way to actually create a PipelineRun without immediately executing it is to leave it in "Pending".

The Tekton UI requires you to open the kebab menu and to select "Rerun" to start it.

Tekton intend to implement a better experience here: RFE-2423

I think we still may want to have something in our own UI to manage the lifecycle however, particularly if we're going to have this concept of a "migration list".

Let's make a decision for this one how we want to progress forwards, or if the Tekton enhancement is enough. If we want to implement something on our side.