Until now, the `<Miltimedia>` component operated on URIs but now encrypted media don't operate on URIs (although for keyserver uploads, the `holder` is a URI on which we rely here).
Replaced URIs with a Source objects which can be either a URI or encrypted media source.
The component uses the departing URI concept to indicate whether a temporary local file can be disposed. In theory, encrypted media don't have to be marked as departing, but I did it anyway for consistency (this should have no negative impact).
Added support for rendering encrypted source by using the `<EncryptedImage>` component added in D7225 is used.
Depends on D7225