install ember-router-scroll, use window scroll position

This installs the ember-router-scroll addon to properly remember scroll
position between routes, and to reset it when the route changes. There
was already a (copy-pasted?) config for this in the dummy app, but it
pointed to an element that doesn't exist. Instead I just removed the
config and it now uses the `window` scroll position instead, which works
well.

package.json

@@ -85,6 +85,7 @@
     "ember-native-dom-helpers": "0.6.1",
     "ember-radio-button": "^1.2.3",
     "ember-resolver": "^4.0.0",
+    "ember-router-scroll": "^0.7.1",
     "ember-source": "~3.1.0",
     "ember-source-channel-url": "^1.0.1",
     "ember-truth-helpers": "^2.0.0",

tests/dummy/config/environment.js

@@ -6,9 +6,6 @@ module.exports = function(environment) {
     rootURL: '/',
     locationType: 'history',
     historySupportMiddleware: true,
-    routerScroll: {
-      scrollElement: '#docs-viewer__scroll-body',
-    },
     fastboot: {
       hostWhitelist: [/^localhost:\d+$/],
     },