Refactor MAL Scraper

There's a few caveats to this refactor:

* Manga won't work until Casting has a polymorphic relation (I rely on it for staff and characters for manga)
* It might lock up the server to download like 40 images from MAL's CDN due to scraping all the character and voice actor images
* Only `mal_import.rb` has tests right now.  I might add some coverage to the `create_or_update_by_hash` methods later.

Definitely triple check that I didn't break anything, because this thing is huuuuuuuuuuuuuuuge