new skin for Nomensa video player

- new CSS
- new images
Use the new player styles everywhere

- Removed the old nomensa player styles
- Moved the new styles into their own helper
- Shared that helper between the html publications and the main
  frontend.
Remove old media player images
Share the new media player CSS with the admin
Merge pull request #674 from alphagov/htmlpub-video-player

https://www.pivotaltracker.com/s/projects/367813/stories/50163659

new skin for Nomensa video player