Remember imported subtitle file

### Describe your suggested feature

When you import an external subtitle file, it remains there only for that session, meaning, the subtitle file needs to be manually imported again every time you close the video player. What I want is the player to be able to remember the last opened subtitle file. Possibly, import it to the app's internal storage for it to remain possible to access it even if it's deleted from external shared storage

### Other details

Suggestion related to #1520 : add an option to set a subtitle folder, which would function similar to local library. Basically, for the corresponding series name, if a folder exists, import all subtitle file from within that subfolder.

### Acknowledgements

- [x] I have searched the existing issues and this is a new ticket, **NOT** a duplicate or related to another open or closed issue.
- [x] I have written a short but informative title.
- [x] If this is a request regarding an extension, I should be opening an issue in the extension's repository.
- [x] I have updated the app to version **[0.16.4.3](https://github.com/aniyomiorg/aniyomi/releases/latest)**.
- [x] I will fill out all of the requested information in this form.