[Variations UI] Filter Variations by Attributes

Originally logged in https://github.com/woothemes/woocommerce/issues/8537 It would be useful to be able to sort and filter variations by their assigned attributes.

e.g. If I had a variable product with attribute "Size", I could filter the list of variations to show only "large" or "small" variations. This would make it easier to work with large amounts of variations at once.

The issue names the Variations UI but no files, tests, or entry points. Locate the JavaScript implementation of the variations list, first tracing any existing sorting or filtering behavior; done means users can filter variations by assigned attributes such as Size=large or Size=small.

Written by the indexing model from the issue text.