Fixing custom_thumbnail updates.

[dessalines]

No description provided.

[Nutomic]

I believe this is still not working correctly. Specifically if you set a custom thumbnail when creating a post, then edit the post body it will overwrite with auto-generated thumbnail. So in post/update.rs if the url is unchanged, you have to do custom_thumbnail = data.custom_thumbnail.or(orig_post.thumbnail). Or in fact it doesnt have to call generate_post_link_metadata() in update.rs if the url is unchanged.

Would be good if you could add an api test for custom thumbnail functionality.

[dessalines]

Found an issue with image posts, I'm checking it out.