Add Avatar Srcsets

[zicklag]

It would be useful to have larger and smaller versions of the avatars available so that you could use an image srcset or specific URL to display an appropriately sized images based on the theme / screen / display size.

[zicklag]

The best option medium-term is probably to cache resized versions of the avatar locally so that we don't have to worry about storing multiple sizes of the avatar in the database.