[⭐FR] Migrate downloaded episodes

### Describe your suggested feature

The only time I would ever migrate, is when some episodes can't be loaded. Downloaded episodes will disappear after migration. In other words, downloaded episodes are tied to their own anime source. I'd like to see downloads carried over so that a whole season can be watched offline (or without any buffering), when some episodes fail on one anime source, but do work on another where other episodes may have problems.

### Other details

This could introduces bugs when migrating to a different anime series with a similar name. It would be best to show an icon indicating that an episode has been downloaded from a different source and be able to remove these downloads from **the series you migrated to**. The downloads will be kept on the source you got them from.

### Acknowledgements

- [X] I have searched the existing issues and this is a new ticket, **NOT** a duplicate or related to another open issue.
- [X] I have written a short but informative title.
- [X] If this is an issue with an extension, I should be opening an issue in the [extensions repository](https://github.com/jmir1/aniyomi-extensions/issues/new/choose).
- [X] I have updated the app to version **[0.12.3.5](https://github.com/jmir1/aniyomi/releases/latest)**.
- [X] I will fill out all of the requested information in this form.