[dagit] Upgrade to React 17
Upgrade Dagit to React 17. This can wait til after the Thursday release.
I fixed a bug that showed up in the logs scrolling table on the Run page, but otherwise I didn't spot any issues. In a followup, I'll clean up peer dependencies mismatches where I can.
Test Plan: Buildkite, load Dagit and click everywhere. Verify that things seem okay.
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D7246