Extract 'image' and 'users' metadata form the post/comment content and include in the API response

Most frontends right now include some additional metadata for posts and comments. Usually at least image and users are provided.

I think will be cleaner to drop those metadata when publishing a post/comment and compute both those fields (array) in Hivemind. The fields will always be provide in the API response as it is right now.

This way Hivemind will not rely on data provided in the operation (that are not guaranteed to be up to date with the post content) and can also provide those field for posts published without including those values (improving consistency).