allow admin to prevent private image embedding on hub

There are some privacy and database size drawbacks to embedding private images in posts. This pull allows the admin of a hub to prevent that embedding from happening.

In order for private images to still display, a function has been added to add redir/ links to all images with Friendica-style photo URLs found in private posts or comments. Note that this functionality can't depend on the option to prevent private image embedding, because it's the *receiving* hosts that need to deal with the lack of embedding.

I've run this on my hub for a while, and it works pretty well (sometimes you need to refresh to see all the private images on a post, particularly if there are a large number attached). I don't know how well it will work between two physically separate hubs. If it doesn't work well in the latter case, it may be more useful for community-based hubs, where people who sign up mostly connect to other people on the same hub. In that case, it will decrease the database size significantly.