[dagit] Fix responsiveness of partition graphs, flex:1 without min-width prevents shrinking
Summary: This is a small patch to addres https://github.com/dagster-io/dagster/issues/3537. I also removed the 1600px max-width because the new run matrix goes full-width at larger sizes and it looked odd with fixed-width content beneath it.
Test Plan: Run tests
Reviewers: dish, sandyryza
Reviewed By: dish
Differential Revision: https://dagster.phacility.com/D6228