Decide when, how, and who shall delete obsolete Git branches

e.g. post-release? post-merge? once a year? the release manager? the sysadmin team? What kind of privileges are needed to do that?

Blueprint: https://tails.boum.org/blueprint/delete_obsolete_Git_branches/

Parent Task: #8617 (closed)

Original created by @intrigeri on 8616 (Redmine)