Changes to Custard as part of move to Wordpress

# Merge once [proxy changes](https://github.com/scraperwiki/charms/pull/20) have been made

Part of https://github.com/scraperwiki/master-repository/issues/25

- [x] Add data-nonpushstate to "professional services" and "table extract" links in Custard `#nav`

- [x] Move "logged in" Custard homepage to scraperwiki.com/datasets (scraperwiki.com should always show the logged-out homepage)

- [x] Fix Custard tests so they work with the new logged-in homepage URL