overhaul showcase

Summary:
The motivation is that the showcase is becoming far too large to be useful. I filtered the apps for, basically, "apps that have some sort of interesting news coverage or technical blog post about them". The UI is a bit updated to also mention something about the information link. I also added the FB app itself.
Closes https://github.com/facebook/react-native/pull/8263

Differential Revision: D3463856

Pulled By: JoelMarcey

fbshipit-source-id: cdd309ba85edca417868f14dee7c772f73af654b

website/src/react-native/css/react-native.css

@@ -1326,15 +1326,10 @@ div[data-twttr-id] iframe {
 @media only screen
   and (min-device-width: 1024px) {
   .showcase {
-    width: 20%;
+    width: 25%;
   }
 }
 
-.showcase:hover {
-  text-decoration: none;
-  opacity: 0.8;
-}
-
 .showcase h3 {
   margin-bottom: 0px;
   line-height: 20px;