Configure a default landing page for each profile

[wolfgangmm]

Feature profiles may not come with a document view, so there won't be anything to see if a user installs just the feature. We should therefore make the landing page (i.e. what serves as index.html) configurable. For example, the registers feature profile could define /people as the default.

Features with a document view, e.g. parallel or monograph would overwrite the landing page to show the browse documents view instead. For a working app you would need a profile hierarchy like ["base10", "registers", "parallel"] and the last in sequence determines what to use.

base10 itself should include a simple about page as landing page, so it can be instantiated and shows something meaningful.