Update models to support filenames

[philipsharp]

The PhotoModel and VideoModel objects do not support filenames. Not all services support filenames for photos and videos, and those that do have inconsistent behavior. (Filenames may be dropped, or end up in different fields on import.) This makes it harder to users to keep track of files across services (and possibly local storage).

Update the models and exporters to support storing an existing filename and update importers to use filenames when available and the importing service supports them.