Add ability to put captions in object storage

Deprecate:
 * `path` and `url` of `ActorImage` (used to represent account/channel
   avatars/banners) in favour of `fileUrl`
 * `path` of `AvatarInfo` (used in notifications) in favour of `fileUrl`
 * `captionPath` of `VideoCaption` in favour of `fileUrl`
 * `storyboardPath` of `Storyboard` in favour of `fileUrl`

server/core/lib/object-storage/keys.ts

@@ -17,6 +17,10 @@ export function generateOriginalVideoObjectStorageKey (filename: string) {
   return filename
 }
 
+export function generateCaptionObjectStorageKey (filename: string) {
+  return filename
+}
+
 export function generateUserExportObjectStorageKey (filename: string) {
   return filename
 }