FIX: avatars in quotes/oneboxes

Avatars in quotes/oneboxes are still pointing to the old
`/users/:username/avatar(/:size)` route.
So, this adds back the old avatar route for the transition period.
Merge pull request #1323 from ZogStriP/fix-avatars-in-quotes-in-already-cooked-posts

FIX: avatars in quotes/oneboxes