Use asset pipeline for jstree

These fixes are required for styles to load correctly on the taxonomy edit page in production mode in Rails 4.

I see the contributing guidelines say that tests are required, but I'm not sure how I might go about testing this. Will give it a go if you have any ideas.

* Hard-coding links to /assets/ without fingerprints does not work on Rails 4
* The version number in the query string is not necessary for fingerprinted assets.