Small design system simplifications

Summary:
So far we distinguished `Layout.Container` from `Layout.Vertical`, but they did almost exactly the same, so figured to unify them, so smaller API ftw :) Normal containers put children vertical, and if you want to use horizontal, use Layout.Horzontal

Also simplified code in Layout file a little bit.

Fixed issue I split container where the main container didn't go "underneath" the fixed container

Reviewed By: cekkaewnumchai

Differential Revision: D24502547

fbshipit-source-id: 517db3692749e670cda8f0cd7cb1c807df818b4d

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

@@ -19,7 +19,7 @@ const {Title, Text, Link} = Typography;
 export default function SandyDesignSystem() {
   return (
     <Layout.ScrollContainer className={reset}>
-      <Layout.Vertical gap="large">
+      <Layout.Container gap="large">
         <Card title="Flipper Design System" bordered={false}>
           <p>
             Welcome to the Flipper Design System. The Flipper design system is
@@ -122,7 +122,7 @@ export default function SandyDesignSystem() {
         <Card title="Layout components" bordered={false}>
           <DesignComponentDemos />
         </Card>
-      </Layout.Vertical>
+      </Layout.Container>
     </Layout.ScrollContainer>
   );
 }