PushState for Modals

[DFurnes]

Modals should support loading in content two different ways:

1. Load in contents of a given URL via AJAX into a modal. If supported, update the page URL in the location bar and let the user hit the back button to get back to the original page. This should pass an X-PJAX header (or equivalent) to toggle whether containing layout is rendered or not.
2. Load in contents from a pre-loaded element in the page by ID in data-id attribute. (current behavior)

If both are specified, load the pre-loaded div contents and use pushState to update the canonical URL without an AJAX request. This lets us "fake" loading the real page so the user can just copy the location in the address bar if they want to bookmark or share the link.