With the existing avatar implementation, users that don't have a usable

avatar service (such as when Gravatars are used but the user has opted

out) would result in an empty space where the avatar would otherwise go.

This could lead to display inconsistencies, unwanted logging, or other

problems.
This change adds a new avatar service that's used as a fallback when no

other service is available. This will show the first two letters of the

username, overlayed on top of a colored background. The background color

is based on the username, helping to provide different visuals for

different users.
Consumers can replace the fallback with some other service by overriding

fallback_service_class on the registry.
To help with styling, both the existing avatars and the new ones have

new namespaced CSS classes (.djblets-o-avatar), which are meant to

replace the .avatar class (though this is still used).

Unit tests pass.
Tested various avatar sizes, making sure the fonts are in proportion.