[dagit] Use tabular-nums spacing on TimestampDisplay

Summary: Tidy up timestamp string rendering a bit by using tabular-nums. I need to spend some time cleaning these up more generally.

Test Plan: View "Upcoming ticks" table on "All schedules", verify that the timestamp column has consistent text.

Reviewers: bengotow, prha

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D5952