[dagit] Repair run tag overrides

Summary:
In cases where a code-defined pipeline tag is overridden via the tag editor:

• Show the overridden tag as being overridden, with a lower opacity and a tooltip
• Ensure that the newly saved tag is actually applied to the run -- this is currently broken, because the updated session is not being applied to the execution button. This is due to a React render short-circuit, which keeps getVariables bound to a stale session object.

Test Plan:
View the playground for a pipeline with a code-defined tag.

• Verify that the tag shows up correctly in the playground.
• Override it with the tag editor. Verify that the overridden tag is displayed accordingly. Save.
• Verify that the overridden tag and the override tag both appear in the tag list on the playground, with the low-opacity/tooltip display on the overridden tag.
• Kick off a run. Verify that the generated run has the overridden tag applied to it.

Reviewers: bengotow, alangenfeld, prha

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D8118