Upgrade to Rocket v0.4.0

Created by: Commander-lol

The latest version of rocket will require a few changes to the internals, but also provides an opportunity to simplify the codebase.

Static file serving is now provided as part of rocket_contrib and can be configured programmatically. This opens up a new config opportunity: mounting multiple different folders under routes. The serve fairing also has all the options required to implement the current serving options (mainly rendering an index.html page if found)