Fix #245 Add a preview and closed date to the site

There are now two extra dates available for a conference - a preview date and a closed date.

In the preview date range, all urls to the site will get redirected to a `/preview` page that shows a holding/teaser page (e.g. coming soon...).

In the closed date range, all urls to the site will get redirected to a `/closed` page that shows that the conference is over for the year.