Upgrade Admin UI to Ember 2

[GUI]

The admin app is currently an Ember 1.7 application living inside the Rails web-app component. It would be good to upgrade this to a more recent version of Ember that has better support these days.

As part of this upgrade, it would also be a good idea to pull the admin app outside of the Rails web-app component and into a standalone Ember CLI app. Now that the admin interface has really turned into an Ember-only app (utilizing the various admin APIs), and now that Ember CLI is the standard way to develop Ember apps, it would be a better fit for future development.

It would also be good to upgrade Ember from a security perspective. For example, the current version of Handlebars.js our Ember app is locked to has a security advisory. While I've audited our code-base to ensure we're not vulnerable to that specific issue, upgrading would be a better long-term fix.

This upgrade is currently in progress on the admin-upgrade branch. In there, the root of the Ember CLI app is at src/api-umbrella/admin-ui.

[GUI]

Landed in #312