Remove BaseURL from Scene struct

This ends up being a bit of a security problem, as scenes files will be
checked into source control and the base URL may be sensitive (e.g.
internal only). So, BaseURL is being removed, and the URI's stored in
each scene will contain only the Path (not the host part of the URL). In
addition:

- If the `baseurl` key is present (in an existing json/yaml file) it
  will be ignored.
- If the `baseurl` key is present and a new scene is added to an
  existing file, the `baseurl` key will be removed.
- Finally, any existing routes will need to have the URL portion of the
  URI removed or they will never match.