add `user.avatar_updated_at` timestamp to comments cache key
add `user.avatar_updated_at` timestamp to comments cache key

makes comment caches obsolete when a user changes his avatar

teambox/teambox,nfredrik/teambox,irfanah/teambox,irfanah/teambox,vveliev/teambox,rahouldatta/teambox,alx/teambox,rahouldatta/teambox,crewmate/crewmate,teambox/teambox,teambox/teambox,codeforeurope/samenspel,crewmate/crewmate,ccarruitero/teambox,nfredrik/teambox,rahouldatta/teambox,vveliev/teambox,nfredrik/teambox,irfanah/teambox,ccarruitero/teambox,wesbillman/projects,codeforeurope/samenspel,vveliev/teambox,vveliev/teambox,irfanah/teambox,wesbillman/projects,crewmate/crewmate,alx/teambox,nfredrik/teambox