[dagit] Preserve solid selection when refreshing config

Summary:
Based on a user report. With the change to allow refreshing configs after a manual repo reload, the user's current solid selection is not preserved.

Fix this by keeping the currentSession.solidSelection around, if any.

Test Plan: View pipelines with presets and partition sets. Use a solid selection, then reload the repo and refresh config. Verify that the solid selection is preserved.

Reviewers: bengotow, catherinewu, sashank

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D6425