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/actions/onboarding.jsx

@@ -0,0 +1,14 @@
+import { openModal } from './modal';
+import { changeSetting, saveSettings } from './settings';
+
+export function showOnboardingOnce() {
+  return (dispatch, getState) => {
+    const alreadySeen = getState().getIn(['settings', 'onboarded']);
+
+    if (!alreadySeen) {
+      dispatch(openModal('ONBOARDING'));
+      dispatch(changeSetting(['onboarded'], true));
+      dispatch(saveSettings());
+    }
+  };
+};