[dagit] Truncate runs on Schedule view
There's no default limit on pipelineRunsOrError, so the Schedule view can end up loading way more runs than we need. Limit to 20 for now. We'll want to figure out a nice consistent pagination/scroll-loading story for these views, but let's just make things less sluggish here at this point.
Also change the header to "Latest runs" to clarify that this is a truncated set.
Test Plan: View schedule, verify that the Runs view is truncated.
Reviewers: dgibson, prha, johann
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D5247