use root_path in api.url

[pmeier]

Supersedes #305. After an offline discussion with @aktech we agreed that it preferable to include the config.api.root_path in config.api.url. While this means we have this we have this information duplicated in the default configuration, this has two upsides:

1. We don't need to join the URL and the root path everywhere in our code base.
2. It opens up the possibility to route an app deployed with a root path to a top level domain without one.

[aktech]

Looks good to me, verified it works.