Replace sprockets/browserify with Webpack (#2617)

* Replace browserify with webpack

* Add react-intl-translations-manager

* Do not minify in development, add offline-plugin for ServiceWorker background cache updates

* Adjust tests and dependencies

* Fix production deployments

* Fix tests

* More optimizations

* Improve travis cache for npm stuff

* Re-run travis

* Add back support for custom.scss as before

* Remove offline-plugin and babili

* Fix issue with Immutable.List().unshift(...values) not working as expected

* Make travis load schema instead of running all migrations in sequence

* Fix missing React import in WarningContainer. Optimize rendering performance by using ImmutablePureComponent instead of
React.PureComponent. ImmutablePureComponent uses Immutable.is() to compare props. Replace dynamic callback bindings in
<UI />

* Add react definitions to places that use JSX

* Add Procfile.dev for running rails, webpack and streaming API at the same time

app/javascript/styles/application.scss

@@ -0,0 +1,20 @@
+@import 'variables';
+@import 'fonts/roboto';
+@import 'fonts/roboto-mono';
+@import 'fonts/montserrat';
+
+@import 'reset';
+@import 'basics';
+@import 'containers';
+@import 'lists';
+@import 'footer';
+@import 'compact_header';
+@import 'landing_strip';
+@import 'forms';
+@import 'accounts';
+@import 'stream_entries';
+@import 'components';
+@import 'about';
+@import 'tables';
+@import 'admin';
+@import 'rtl';