This adds a medium (500px) size avatar thumbnail, that can be
referenced as `{name}-medium.png`. It is intended to be used on the
user's own settings page, though we may come up with other use cases
for high-resolution avatars in the future.
This will automatically generate and upload the medium avatar images
when a new avatar original is uploaded, and contains a migration
(contributed by Kirill Kanakhin) to ensure all pre-existing avatar
images have a medium avatar.
Note that this implementation does not provide an endpoint for
fetching the medium-size avatar for another user.
[substantially modified by tabbott]
Brock Whittaker