Add per-level autoplay videos and associated level editor field

- Add level editor field for autoplay video
- Add video preview to level editor

![image](https://cloud.githubusercontent.com/assets/206973/3885203/3ff3ad64-21bd-11e4-86a5-3f0e4044fdac.png)

- Add "Coming soon" message for videos without text slides

![image](https://cloud.githubusercontent.com/assets/206973/3885217/7f695372-21bd-11e4-9fd1-93d17f2c088c.png)

- Don't auto-play `concept`-keyed videos in non-legacy levels
- Rename `@videos` to `@related_videos`
- Add tests for related videos, autoplay videos
- Test concept importing
can shorten to `Video.all.map(&:key)`