src/{ui,intro,constants}.js: Use Youtube embed for intro video.
src/{ui,intro}.js: Youtube video: Better error handling.
- Give it a second to load before falling back to local video file.
- Add onError event handling.
src/co{nsta,mpone}nts.js: Rename 'Image' to 'Img', avoid clobbering existing component.
src/constants.js, img/logo.png, .gitignore: Replace assets/ with img/ and video/