Fix slideshow arrow colors in Facebook Conclusion - Bug

[RahulKhanna14]

In the conclusion frame of the Facebook Ads case study, the blue arrows to navigate between the cards work as intended, but we'd like for the arrows to turn gray if they shouldn't be clicked. Specifically, the left arrow should be gray on the 1st card (can't go left of the first) and the right arrow should be gray on the 3rd card (can't go right on the last).

(In the below image, right arrow should be gray since we shouldn't be able to click it)

The relevant code is in website/src/components/posts/Facebook/conclusion/ConclusionGraphic.js. There seems to already be some pre-existing code that tries to achieve this, but it currently does not work.