Enhancements for the media-processing panel

[Olivier Mehani]

As mentioned in #463, some ideas for things to add to the control panel:

• Queue of media in the unprocessed state
• Allow to kill and restart processing for media in the processing state (they sometime get stuck) [aka `update coremedia_entries set state='unprocessed' where id='ID' gmg reprocess run ID initial`]
• Allow to reprocess processed entries [aka `gmg reprocess run ID transcode`]
• Allow to regenerate the thumbnail for one entry [close to, but not exactly, gmg reprocess thumbnails]
• Allow to rotate media, see #5487

[ayleph]

I created a separate ticket for showing media in the unprocessed state in user panels. See #5564.