rename import `classnames` to `classNames`

Created by: 5h1rU

Imo classNames makes more sense because that's how it's capitalized in the DOM/React API (and in English, it would be two words).

But +1 to a lint rule for this, pointing this out on PRs and changing it back and forth is not a good use of our time :)

Originally posted by @felixfbecker in https://github.com/sourcegraph/sourcegraph/pull/21474#discussion_r645389897