Replace media-player styles

Replace styles with @MarkHurrel's redesign from alphagov/whitehall
Remove old media player images

These are no longer used following the redesign in 6e99bcf6
Remove media-player assets

As the media player is shared across apps it now lives in static
Remove media player references

The CSS & javascript are now in the static application files & no longer need adding to templates manually
Use video styles from toolkit

- Re-add the media-player-loader JavaScript. Initalization should live
  with each app customised for the app
- Bump the toolkit so the video Sass files are available
- Use the video player sass files
Merge pull request #418 from alphagov/marks-media-player

Remove media player