Prevent using URL of the same site

[cr0ybot]

The core RSS block prevents pulling in a feed with the same URL as the current site. I think this is probably wise, but I'd like to offer a filter to allow it for some advanced usage that I might not fully grasp at this point.

This should probably be handled by the FeedBlock\Feed\get_feed function so that it can be filtered once in PHP. Will have to consider what kind of return value (WP_Error?) would make the most sense and how that is handled both in editor and front end.