Update logic to cache thumbnails for Collection #42

For the newest series, Kitsu is no longer always sticking to the strict URL format for poster images that they have in the past.

When adding items to the collection, the image should be saved and cached, instead of relying on the implicit method currently used.

For the newest series, Kitsu is no longer always sticking to the strict URL format for poster images that they have in the past. When adding items to the collection, the image should be saved and cached, instead of relying on the implicit method currently used.

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Update logic to cache thumbnails for Collection #42