Use babel-polyfill in place of polyfill.io CDN (#2496)

Update provides support for running the examples app on IE 11.

There appeared to be a number of issues with using `polyfill.io`
with slate. While trying to run the example app on IE 11, I ran
into each of the following:
https://github.com/Financial-Times/polyfill-service/issues/792
https://github.com/Financial-Times/polyfill-service/issues/854
https://github.com/Financial-Times/polyfill-service/issues/1341

`babel-polyfill` provides more robust support for polyfilling and
will allow for better targeting of supported browser envs in the
future (if desired).

Also includes `element-closest` polyfill, which is necessary for
using with IE 11 :(

package.json

@@ -12,6 +12,7 @@
     "babel-plugin-external-helpers": "^6.22.0",
     "babel-plugin-module-resolver": "^3.1.1",
     "babel-plugin-transform-runtime": "^6.23.0",
+    "babel-polyfill": "^6.26.0",
     "babel-preset-env": "^1.6.1",
     "babel-preset-react": "^6.24.1",
     "babel-preset-stage-0": "^6.24.1",
@@ -22,6 +23,7 @@
     "copy-webpack-plugin": "^4.4.1",
     "cross-env": "^5.1.3",
     "css-loader": "^0.28.9",
+    "element-closest": "^2.0.2",
     "emojis": "^1.0.10",
     "emotion": "^9.2.4",
     "eslint": "^4.19.1",