[Front page] Update the feature list and separate access checking

[rikurauhala]

On the front page, there is a list of features and a brief description for each of them. However, some of them are missing.

There is also logic for checking which features the user has access to. The access checking logic could be extracted and the same logic should be used for both the feature list and displaying the actual navigation buttons in the navigation bar. If the user can access a feature, they should see the navigation link and the feature description.

See NavigationBar and FrontPage.

Access right testing could also be improved. Tests should include visibility of each navigation button and checking whether the user can acually access the page or not (access denied).