Towards #1415.
Potential candidate for a web-based world display
Uses [pixi.js](https://pixijs.com/).
# Demo
1. Run swarm: `scripts/play.sh -i scenarios/Fun/horton.yaml --autoplay --speed 6`
1. Visit http://localhost:5357/play.html
This is in preparation for both #1672 and #1650.
* Added some utility methods for color flattening
* The `Grid` type has a new home in `Area.hs` and now derives more instances
* `getTerrainEntityColor` is extracted from the `getDisplayColor` function
* Add a favicon to silence browser warnings
API docs are moved under `api/`, and the landing page for http://localhost:5357/ is now a static `index.html` page.
This paves the way for a JS-enabled web frontend demo.