Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/proprietary/hooks/useRequestHeaders.ts

@@ -0,0 +1,4 @@
+export function useRequestHeaders(): HeadersInit {
+  const token = localStorage.getItem('stirling_jwt');
+  return token ? { 'Authorization': `Bearer ${token}` } : {};
+}