ThumbnailVideo: rewrite URLs

We need to modify the way URLs for shared repository videos are generated. Currently ``wfReplaceImageServer`` is called which uses current aka local wiki setup - including image domains.

However, during migration to distributed file system we need to keep shared videos on old storage for a while, but "force" migrated wikis to use old images domain.

@garthwebb - please review

Original pull request: #1746