Detect and "fix" (transcode) VBR mp3s

[cavis]

Our waveform editor has some issues with VBR mp3s. Which makes marking ad-breaks tricky.

When a user uploads a VBR MP3, we should transcode it to CBR.

• Make Porter return some indication that the file was VBR - easiest just to check the mpck output
• Feeder should check that flag and fire a fix media task
• The fix-media-task would also have to re-encode that file to CBR for these cases (slightly different command).

I think it makes sense to build this into the existing "fix media" jobs? And to do this for both Uncut and multi-segment Contents? But I could be convinced otherwise.