Home URL

Formerly the home button in the navigation bar simply changed to the entry's first page. This button is now called `topButton` and has an up arrow icon in the default theme. This pull request also adds a new home button, which is supposed to link to some kind of external overview page. This so called `home_url` can be configured in the theming, but can also be overridden on a per entry basis. The `home_url` is also used to redirect requests to the public root path based on the theming's cname if configured.