command to cleanup old subset pipeline branches

glp subset can leave lots of temp branches over time. We need to clean them up, and warn when there are too many.

glp subset --clean should delete all of the current user's subset branches glp subset --clean --all=24h should delete all of subset branches (created by any user) that are older than 24hrs

glp subset should also occasionally warn the user if there are a large number of old subset branches