This commit removes the `html' package dependency, speeding up the
compile time of the Dart code.
It also simplifies the code and removes some unused code. For instance
the sun, moon, and lightbulb icons is removed, the `Theme' class is
removed, and the `switchTheme' function is now merged into the
`setTheme' function. The `makeThemeItem' function has also had its
second argument removed.
Sitemap generation is now independent from the navbar generation, and
it will be added to the navbar only once the navbar has been created.
Also, better style for code.
