[UX] Allow for running of sub-boards and nodes

[paullewis]

I was making some tools today, and a couple of them had sub-boards. We don't currently support running sub-boards, but I think it would be a good thing to have so you can check it does the right thing in isolation.

@dglazkov I think I might need you for this a bit. Do we need to anything in particular to make it work from a runner perspective? I feel like we could either support subgraphs in the runner, or we could make a new runner instance when we want to run the subgraph... but also I feel like this has implications for running a node in isolation, too. Is there some unified way of approaching this?

[dglazkov]

Need to decide on an approach here: I wonder if we auto-create a tiny board for a node to run it? This way, we could have a separate history of runs, but could merge in the inputs from existing runs, etc. But then we'll need a separate UI treatment for the one-node-board.