Improve markdown header spacing

Summary:
The spacing above headings didn't look right.

Also workplace purposely limits the different heading sizes to 2 to encourage using notes for large posts. So I'm changing this to match. If we need to have arbitrary depth in future, we can add a `workplaceMode` toggle, but so far that's the only use case we have.

Reviewed By: mweststrate

Differential Revision: D18638861

fbshipit-source-id: 3949a37515e4282bb8435c2740ef45a55e5a97cc

src/ui/components/Markdown.tsx

@@ -19,7 +19,8 @@ const Row = styled('div')({
   marginBottom: 5,
 });
 const Heading = styled('div')((props: {level: number}) => ({
-  fontSize: Math.max(10, 20 - (props.level - 1) * 4),
+  fontSize: props.level === 1 ? 25 : 18,
+  marginTop: 10,
   marginBottom: 10,
   fontWeight: 'bold',
 }));