Switch the release process to a single zip file

1. Only generate a single `Cesium-xxx.zip` file which basically includes the contents of the old `full` versions along with the below changes.
2. Move `server.js` to the top level folder
3. Merge multiple `index.html` files into a single, top-level file.
4. Remove Timeline demo from `index.html` and zip file.
5. Add descriptions and improved styling to `index.html`
6. Add link to WebGL report to the home page

I still want to clean up the styling/layout of index.html further, but feedback and suggestions are welcome.