[dagit] Settings page

Summary:
Restore the Settings page originally created for the earlier left navigation experiment. This gives us a place to:

• Move the timezone selector, which occupies valuable left nav space in spite of being a one-time setting for users
• Surface feature flags for enabling/disabling features without our needing to direct users to find /flags

Test Plan: Load dagit, verify that the settings cog renders and behaves correctly in the top nav. View Settings, verify that everything behaves correctly when clicked. View left nav, verify that scrolling is correct, inside and outside of Crag flag.

Reviewers: bengotow

Reviewed By: bengotow

Differential Revision: https://dagster.phacility.com/D8567