Add infinite scrolling to admin measures view

This starts a move for the Admin View toward a more Backbone-centric view. I removed inline jQuery AJAX calls and replaced them with Thorax models, which automatically manages the fetch lifecycle.

This still needs some additional cleanup in the future... the [existing code](https://github.com/pophealth/popHealth/blob/master/app/assets/javascripts/views/admin/measures_admin_view.js.coffee#L115) is using [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData) for interacting with a form, which doesn't work in IE9; instead, we should use [serialize](http://thoraxjs.org/api.html#serialize)/[populate](http://thoraxjs.org/api.html#populate) instead, which are built in to Thorax.