"Site Not Found" redirect

[dhedlund]

When a site can't be found, it redirects to the activatehub.org wordpress site which can be confusing.

1. Create a new site_not_found view to display when the domain part of the site doesn't match what was requested, such as when a city is mistyped ("protland.activatehub.org")
2. For cities that have multiple calendars, if the domain part of the site matches what was requested but a calendar wasn't found, it should redirect the the city's primary calendar and display a message that the calendar wasn't found. The primary calendar is the site that matches the domain but with an empty Site#path_prefix value. Assuming an "arts" calendar, if someone visits http://sfbay.activatehub.org/art, it should redirect them to http://sfbay.activatehub.org/ and display a message that says the calendar wasn't found. The city's landing page will eventually include links to all of it's calendars.