Add list of batch specs to details page

Created by: eseliger

This PR ticks off the last checkbox from the EM3.1 milestone. I added a basic list based on what we already had in the site-admin and incorporated some of our ideas around this page. It's not super polished yet, but it'll serve the purpose until we have more detailed designs for this page. It's possible to get to all executions associated with the batch change, see execution statuses and find the current spec, as well as quickly looking at the spec inputs. This menu is not available when the feature flag is turned off. It's meant as a replacement for the existing spec tab.