Correct the backdrop url and path

This makes the backdrop base url and path work across development,
preview and production.

- Configure backdrop base url in an initializer so that it can be easily
  change between organisations.
- Update request path to match where the api is being served from. This
  needs improvement. Ideally configuring a pattern in one place and then
  using it everywhere.
Merge pull request #15 from alphagov/correct-backdrop-url-2

Correct the backdrop url and path