make default avatar use the avatars table

apps/users.lua

@@ -150,8 +150,10 @@ app:post("user_clear_avatar", "/:username/clear_avatar", function(self)
   if me.id ~= target_user.id then
     return {redirect_to = self:url_for("user", {username = self.params.username})}
   end
+  local old_avatar_id = target_user.avatar_id
+  util.destroy_avatar(old_avatar_id)
   target_user:update({
-    avatar_id = db.NULL,
+    avatar_id = 1,
   })
   util.inject_infobox(self, "Avatar cleared.")
   return {redirect_to = self:url_for("user_settings", {username = self.params.username})}