Add Previous & Next posts to Model(s)

[MattDrivenDev]

This will enable us to put links on the blog post pages that are:

Previous: Post abc | Next: Post xyx

Would be nice to use following (or similar) Razor to do it:

<a href="/@post.PreviousUrl">@post.PreviousTitle</a>

Would have to figure a nice way to handle it being None or null or whatever.