Serve own avatar from its own endpoint

[kevinmcconnell]

This allows us to have different cache controls depending on whether you're viewing your own avatar, or someone else's. The image that's shown is conditional to the current user, via CSS, so we can continue to reuse the cache fragments. The general avatar endpoint has a 30 minute cache expiration, while the current user endpoint is e-tag only.

The aim here is that your own avatar will always be fresh, while other folks' avatars can be served from CDN.

cc @flavorjones @monorkin