option to include/exclude this post type in normal "river" view of stories

[aschweigert]

I like the idea of being able to include this post type in the regular "river" views in Largo but we'd need to think about styling to make sure it's clear what's a link to an external site, etc. and add some controls to determine where these posts would be allowed to display, taxonomies available to them, etc.

Moved from: https://github.com/INN/argo-links/issues/12