Fix layout of beta banner to better work on the redesign.

- Move the banner lower, so it doesn't overlap with the header
  navigation on narrower screens and also to make it look more related
  to api.data.gov
- Adjust styles and colors so it's fits a bit better with the flatter
  redesign.
Add link to Data.gov's CKAN API and try to clarify this service.
Add link to FBOpen's API since it's using this service.
Initial signup form implementation as an client-side embeddable widget.