Refactor taxonomy js tree

As we discussed in #2192, I re-did my changes on the split_core branch.

You've been doing a lot of great work! 

I only needed to change a couple of lines on this branch to get the desired functionality: I wanted to be able to re-use the taxonomy tree editor elsewhere, which basically just required turning it into a function which accepts `taxonomy_id`, instead of having it load automatically based on the global `taxonomy_id` variable.

(On my original PR, I made a lot more changes, but looks like the Spree::API stuff takes care of all that now).

One last note: has the procedure for running the tests changed? I'm confident this change doesn't break anything in admin/taxonomies/edit, but I don't know if there were other places that used the code, because I couldn't get the tests to run.

I generated the sample app and ran `bundle exec rake spec` (and tried `bundle exec rspec` too), with no results. Also, the generated sample app doesn't seem to be there, even though it said it was generating and took a while to run.