Fix pluralization in biggestfan insight.

Nitpick: Let's not use the "They favorited/liked" body copy when there's only one biggest fan. The only body copy option should be "Here's who favorited/liked" etc. copy.

(I know "they" is often used when you don't know the right gender pronoun, but it is a little weird and since we have another option, let's just use it instead.)