campaigns: spurious Percy failures caused by the landing page video

Created by: LawnGnome

Percy doesn't like the YouTube video on the campaigns landing page very much: it's fairly common for the embedded thumbnail or text to change very slightly and cause a failure. Sample failures:

• https://percy.io/Sourcegraph/sourcegraph-storybook/builds/6112805?utm_campaign=Sourcegraph&utm_content=sourcegraph-storybook&utm_source=github_status_public
• https://percy.io/Sourcegraph/sourcegraph-storybook/builds/6112699?utm_campaign=Sourcegraph&utm_content=sourcegraph-storybook&utm_source=github_status_public

We should use Percy specific CSS to hide the video on that page.