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 fallback for missing video slides

![image](https://cloud.githubusercontent.com/assets/206973/3885206/4a60d6f0-21bd-11e4-9a2b-387de56ab5c9.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