Plan/AC
- Add a feature manager class to the PHP codebase
- Provide a mechanism to register features, e.g. $wgMFFeatures config var
- Add classes to the body element for the CSS code to target
- Forward additional state to the client (via mw.config presumably)
- Add a feature manager class to the JS codebase
- Hydrate the state of the JS feature manager from the forwarded state (from mw.config presumably)