Performance while scrolling in a feed is bad (stutters)

Since we don't know the size of images before loading them, when they finish loading the recyclerview has to move everything around, which makes things stutter and jump around.

We need the API to give size info (like the meta field in mastodon: see https://docs.joinmastodon.org/entities/attachment/)

It would be nice if we could use the blurhash value too (so loading seems more smooth, see https://github.com/woltapp/blurhash), but dansup has been quite clear that this is not in the cards and it is now implemented in pixelfed 🎉