Add some feedback while Cauldron is getting the list of repos for an owner
When adding data sources, sometimes Cauldron takes some time while querying the GitHub/GitLab API. While that happens, the user can only see some "Adding XXX" messages, but no indication Cauldron is working in the background, which may make people think the report is not being produced. We would need some kind of feedback for the user there. I suggest something like "List of repositories being produced, please wait", while all the composing of the list of repos is done.
See #650 (closed) for a report on this lack of feedback to the user.