Subscriptions have no image

Podcast RSS provide images for the subscriptions. The medialibrary should store and expose this to the client in an URL form.

changed milestone to %0.13.0

added feature label

mentioned in merge request !743 (merged)

closed with commit a7b48d6b

closed with merge request !743 (merged)

mentioned in commit 6860fb21

mentioned in commit cdb6da82