Backend: Wait until FFMPEG has released a file before attempting to serve it to the user

[GOATS2K]

When a user toggles off transcoding, we run into a 500 error where the backend tries to read the file before FFMPEG has released its handle. We can either choose to wait for it to complete by waiting x time until we don't get an exception anymore, or we can take the job ID of the last transcoder job and check for completion.