Add import rate and completion estimate to registry migration dashboard
This MR adds an "hourly import rate" and "days to completion" stats to the registry migration detail dashboard. Additionally, it does some minor cosmetic/organization improvements, mostly to adjust graph legends.
Temporary dashboard for demonstration: https://dashboards.gitlab.net/d/DPBcV8Qnk/migration-detail-2?orgId=1
Edited by João Pereira