Add clear diff button, style recording button

Summary:
* Added a button to clear Diff selection
* Added icons and color to the recording button
* Made App input field readonly

Reviewed By: priteshrnandgaonkar

Differential Revision: D18748538

fbshipit-source-id: 00ffb5d5c36f8f5394a353602f182aabd8237ee7

src/ui/components/HBox.tsx

@@ -42,9 +42,8 @@ const HBox: React.FC<{
     shrink: 0,
   };
   const growStyle = {
-    width: '100%',
-    shrink: 1,
-    grow: 1,
+    flexShrink: 1,
+    flexGrow: 1,
     display: 'flex',
     flexDirection: 'column',
   } as const;