Build `Podlove Publisher => Radiator` Migration

[eteubert]

Even when coming from Podlove Publisher, the first step should be to use the feed based importer. However, there should be an additional migration component if the origin is Podlove Publisher, to migrate as much data as possible.

• [ ] Contributors
• [ ] Transcripts
• [ ] Analytics (ideally the raw download intents, if we can manage to keep the connection between original Publisher media_file_id and Radiator file_id)

An API on the Publisher side might be the best way so Radiator can ask for pieces of information at a time and doesn't need to handle possibly huge file sizes (thinking especially about raw tracking data).