Add Login Component

Summary:
- Add Sandy renderer for `SignInSheet`
- Change background color for default button

Reviewed By: mweststrate

Differential Revision: D24112100

fbshipit-source-id: a602c920c24dd039697834aaba3c79e79f04481e

desktop/app/src/sandy-chrome/SandyDesignSystem.tsx

@@ -69,6 +69,7 @@ export default function SandyDesignSystem() {
           <ColorPreview name="disabledColor" />
           <ColorPreview name="backgroundDefault" />
           <ColorPreview name="backgroundWash" />
+          <ColorPreview name="buttonDefaultBackground" />
           <ColorPreview name="backgroundTransparentHover" />
           <ColorPreview name="dividerColor" />
         </Card>