Get Ember Admin ready for production

Closes #3161
- Add a config.js file for the client which is used to configure
  Ember.Application during runtime. The correct version of config.js
  is copied into place by grunt via the copy:(dev|prod) task from
  either config-dev.js or config-prod.js.
- Serve minified and production versions of libraries where applicable
  including handlebars-runtime and ember-prod.
- Bundle third party libraries into vendor.min.js.
- Bundle Ghost's Ember app and templates into ghost.min.js
- Remove all fixture data and code from the client.