Refactored backend.

- Add versioning to API endpoints.
- Refactored $db and $repos as global variables.
- Extracted routes into separate components with deferred registration.
- Support for a different db URL for location data than the one used by
  the application.
- Added sqlite and passport dependencies (passport will soon be used to
  handle authentication).

src/types.ts

@@ -0,0 +1,3 @@
+type Optional<T> = T | null | undefined;
+
+export { Optional };