Complete final prep tasks prior to v2.0 final release

* [ ] Fix remaining HTML, CSS and JS validator issues
* [ ] Update the test site to take full advantage of the new lektorproject and theme options
* [ ] Update the readme with the new theme parameters and other changes
* [ ] Update other release files:
    * [ ] Contributing guide
    * [ ] Roadmap
    * [ ] Changelog
    * [ ] Version file for the release