Rewrite config to use figment

[amouat]

With Trow moving to use Rocket 0.5, the way configuration is handled has changed. Rocket now uses the much more flexible figment system: https://docs.rs/figment/0.10.6/figment/index.html.

We can really clean up a lot of needlessly confusing code and also support of config file by taking advantage of figment. Basically the same figment config would be passed to the Trow frontend, Rocket and the Trow backend. The Trow frontend would need to add some config for Rocket. Each step would just pull out the keys they need and ignore the rest.

As a second step it should be trivial to add support for an external YAML/json file holding config.