lemmy_server should run a SHA256 checksum against every uploaded image and keep a database of all known images so that it does not have to store and link to exact uploads of the same image

There are users who are going to upload the same reaction images and content to comments all over the place. And those users will be on their same home server. Increases performance for all the people reading the comments. Reduces storage for the server operator at the expense of some extra calculations.