media assets: move more file-handling logic into MediaAsset.

Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.

app/logical/storage_manager/rclone.rb

@@ -35,6 +35,6 @@ class StorageManager::Rclone < StorageManager
   end
 
   def key(path)
-    ":#{remote}:#{bucket}#{path}"
+    ":#{remote}:#{bucket}#{full_path(path)}"
   end
 end