Allow bulk deleting of branches

ZD ticket: https://gitlab.zendesk.com/agent/tickets/69328

From the customer:

Is there a way to delete unused/old branches in mass? So far I only see a way to delete them one at a time. Our main repository contains over 2000 branches, many of them are more than 2 years old. This work is no longer relevant and should have been merged into production long ago.