Add back in the custom endpoint, but with better default value

[AramZS]

The most recent update removed the ability to set a custom endpoint on the feed level in order to bring canonical for FB into order. However, I think there is value in maintaining the functionality for various testing, development and internal review purposes, as it allows us to give access to the rough format of an FBIA article without adding additional people to the Facebook page account.

However, I agree that it shouldn't be setting up the wrong canonical by default. I am submitting a pull request with what I think is the best way to resolve this.