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/mastodon/components/column_back_button.js

@@ -0,0 +1,32 @@
+import React from 'react';
+import { FormattedMessage } from 'react-intl';
+import PropTypes from 'prop-types';
+
+class ColumnBackButton extends React.PureComponent {
+
+  constructor (props, context) {
+    super(props, context);
+    this.handleClick = this.handleClick.bind(this);
+  }
+
+  handleClick () {
+    if (window.history && window.history.length === 1) this.context.router.push("/");
+    else this.context.router.goBack();
+  }
+
+  render () {
+    return (
+      <div role='button' tabIndex='0' onClick={this.handleClick} className='column-back-button'>
+        <i className='fa fa-fw fa-chevron-left column-back-button__icon'/>
+        <FormattedMessage id='column_back_button.label' defaultMessage='Back' />
+      </div>
+    );
+  }
+
+};
+
+ColumnBackButton.contextTypes = {
+  router: PropTypes.object
+};
+
+export default ColumnBackButton;