More useful stage dependency graph

[yukw777]

Is your feature request related to a problem? Please describe.
Currently, the flambe report webpage displays a dependency graph of all the stages in the pipeline. While it looks cool, once you have more than three stages, it's pretty much impossible to read it.

Describe the solution you'd like
I think a simpler tree like you see on CircleCI and other CI/CD websites are a lot easier to read and more useful.

Describe alternatives you've considered
I don't pay attention to the graph b/c it's too complex to understand anyway.

Additional context