unbreak dagit read-only mode
Summary: While testing some read-only related changes it became clear that a bad refactor caused the read_only param to the dagit CLI to get thrown out :( This diff threads it through correctly.
Test Plan: BK, load dagit with --read-only and verify you can't launch pipelines
Reviewers: alangenfeld, johann, jordansanders, rexledesma
Reviewed By: rexledesma
Differential Revision: https://dagster.phacility.com/D8811