Onboarding modal (#1883)

* Basic onboarding modal that's shown to users once

* Lay out pages 2 through 5, add images, style modals (#1509)

* Lay out pages 2 through 5

Added images and laid out pages 2 through 5 in the jsx file. SCSS will
come, still working on just seeing if this works at all.

* Fix jsx errors, add images to modal pages, style modal pages

* Add animations to onboarding pager changes, improve wording and styling

* Finishing touches on the onboarding

* Add missing propTypes

* Update wording

app/assets/javascripts/components/features/ui/components/modal_root.jsx

@@ -1,11 +1,13 @@
 import PureRenderMixin from 'react-addons-pure-render-mixin';
 import MediaModal from './media_modal';
+import OnboardingModal from './onboarding_modal';
 import VideoModal from './video_modal';
 import BoostModal from './boost_modal';
 import { TransitionMotion, spring } from 'react-motion';
 
 const MODAL_COMPONENTS = {
   'MEDIA': MediaModal,
+  'ONBOARDING': OnboardingModal,
   'VIDEO': VideoModal,
   'BOOST': BoostModal
 };