SSBC: Replace empty changesets list view for drafts

Created by: courier-new

It's possible to navigate to the BC details page (changesets list) before any changesets have been created. The current empty list view makes it sound like something went wrong ("your query may be too narrow... your steps may not do anything...") since pre-SSBC, this page was inaccessible until after a batch spec is applied. Instead of the current empty list view, we will show a message indicating if an execution is happening.

Design: https://github.com/sourcegraph/sourcegraph/issues/31419. Figma: https://www.figma.com/file/cTKjwbvWqU6xAykZmCTTqy/SSBC?node-id=2160%3A65223