Related Posts: add a default image option

With related posts, one of my biggest reasons for not using it has always been that I think it looks pretty bad when a post doesn't have an image to showcase in the related posts output.

Jeremy Herve just [shared a snippet](http://jeremy.hu/jetpack-missing-images-related-posts/) to enable default images in the module. I think that this is a great candidate for a new conditional option.

Currently, the user can click a checkbox to enable images for related posts output. I would propose that an image upload option conditionally appear upon the image option being selected.

That keeps the option out of sight for folks that don't use images for related posts but makes it easier for normal folks to upload a default image -- what I'd imagine would be an incredibly well adopted feature.

Start by tracing the Related Posts module's existing checkbox for post images and the related-post output, then review the linked default-image snippet for expected behavior. The work is done when users can upload a default image through a conditionally shown option and posts without an image use it in the related-posts output.

Written by the indexing model from the issue text.