Move exploration of definitions to Graphs tab of Repository

Summary:
This work is all behind the tuples feature flag. We conditionally hide the definition tab of the pipeline navigation, and add a new “Graphs” tab at the workspace level. The graphs tab shows composite solids as well as pipelines, and clicking down into one takes you to the pipeline explorer mounted at /workspace/repo@location/graphs/...

Caveats:

• If you click to explore a composite solid, it shows you the first invocation of the composite solid because we can't currently retrieve their graphs outside the context of a pipeline. (TODO: Fix that the pipeline name is still shown in the title)

• There's some slight scrolling wonkiness to the explorer

• Everything is still named "PipelineExplorer" because I didn't want to wreck the diff

Test Plan: Run snapshot tests, poke at changes behind feature flag

Reviewers: dish

Reviewed By: dish

Differential Revision: https://dagster.phacility.com/D8380