Use Node, Class, and Group names as the url slugs

The default URLs will be names, rather than numeric ids. No name history is provided, so renaming a group will break the previous-name URLs (however not the numeric URLs). There are gems to provide slug history, but I don't think that's a required use case for Dashboard.

The old behavior is available by adding `numeric_url_slugs: true` to settings.yml.

I'll add some specific `link_to` tests in a second commit.