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/views/home/index.html.haml

@@ -1,6 +1,6 @@
 - content_for :header_tags do
   %script#initial-state{:type => 'application/json'}!= json_escape(render(file: 'home/initial_state', formats: :json))
 
-  = javascript_include_tag 'application', integrity: true, crossorigin: 'anonymous'
+  = javascript_pack_tag 'application', integrity: true, crossorigin: 'anonymous'
 
-= react_component 'Mastodon', default_props, class: 'app-holder', prerender: false
+.app-holder#mastodon{ data: { props: Oj.dump(default_props) }}