Keep previous avatar images, make it easy to pick an old one again

Currently if you upload a new avatar, your old avatar is gone; to switch back to an old avatar you need to find and re-upload the original file.

It would be nice to keep past avatars in the system to make it easy to switch back; a list of old ones with thumbnails (and maybe upload date?) on the avatar page would make it easy to re-select an old one.

See also ticket #1791 which proposes keeping old avatars around to continue to show the old ones in the timeline.