This commit introduces the following changes:
- Adds an `en_youtube_url` attribute to the `Crop` model to store a URL for an English language YouTube video.
- If a `en_youtube_url` is present for a crop, the video is embedded on the crop's show page.
- A link is added to the "Learn more" section of the crop's show page to search YouTube for "growing [crop name]".
- A helper method is added to extract the video ID from various YouTube URL formats.
- A validation is added to the `Crop` model to ensure that the `en_youtube_url` is a valid YouTube URL.