Don't add duplicate hangers now that closet import can specify a list

Bug report that this resolves:

...However, when I was using the "Import from SDB" tool just a few
minutes ago, it ended up adding EVERY neocash item into the "Not
In A List" section, regardless if I already that item imported
into my "Your Items". So, basically.. I had duplicates of
everything and it would not allow me to move them around into
separate catergories or anything. I know that every other time i've
used the import tool, it would only add NEW items that are not
currently already in my lists yet.
Remove redundant queries when importing closet pages

Specifically, we were running a find_or_initialize_by for all 50
hangers, which isn't great. Collation logic is more complicated this
way, but query count is way lower.

Additionally, compare against hanger.list_id instead of hanger.list,
because hanger.list will fire a query if list_id is non-nil, but that
nil ID tells us everything we needed to know, anyway.