Implement NotFound page type

[HeyJoel]

The Not Found page type does not work and is a hang-over from earlier versions of the closed source product. We should investigate the best way for it to work and implement it properly.

We need to be able to:

• Create content managed, locale specific not found pages
• Create not found pages at any point in the directory structure, e.g. /products could have a different not found page to the root one.
• We should validate that there is only one not found page per locale per directory
• Not found pages shouldn't require a Url
• Investigate if this is also a requirement for error pages? If so, setup another issues for it.