Added IsBlockPreview

[AaronSadlerUK]

Added IsBlockPreview() extension.

There might be a better more fool proof way to do this check, but I've made it check that the request url contains blockpreviewapi

[rickbutterfield]

Hey @AaronSadlerUK! Thanks for this. I had intended to add something like this but wanted it tied in to work similar to how IsBackOfficeRequest() and IsFrontEndRequest() work already in Umbraco, hooking into HttpContext. So I've added your work to the extension you can see in dbb1012!