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.
Currently, this file uses tabs instead of spaces.  I'm fine with switching if you reformat the rest of the file, and add the info about changing tabs to spaces in the Contributor's Guide.