Improve the Variations Metabox Performance

As pointed on #5036 the Variations metabox requires some love towards the performance side of it :smile:.

So my goal with this Pull Request is to improve this metabox by using a bit of JavaScript power to allow the page to have less HTML DOM elements on it.

I've been able to accomplish 2 things already, and there ~~is one~~ are two missing:
- [x] Load the Variation form with AJAX only when needed
- [x] Use Chosen to dynamically create the Select options for each variation attribute
- [ ] Allow the Bulk Actions to be used with this new Structure
- [ ] "Toggle All" link in one single AJAX request

This pull request is still missing a important part of it, but it already show a BIG performance improvement, I will continue to work on this Pull Request to make it work with all the current features of WooCommerce.

---

I would like to ask for some people to help me test it, as it's a big change it should be tested on multiple environments.

cc: @scrivvles @yratof @Reberholt @mxpimp47