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/features/notifications/components/clear_column_button.js

@@ -0,0 +1,27 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { defineMessages, injectIntl } from 'react-intl';
+
+const messages = defineMessages({
+  clear: { id: 'notifications.clear', defaultMessage: 'Clear notifications' }
+});
+
+class ClearColumnButton extends React.Component {
+
+  render () {
+    const { intl } = this.props;
+
+    return (
+      <div role='button' title={intl.formatMessage(messages.clear)} className='column-icon column-icon-clear' tabIndex='0' onClick={this.props.onClick}>
+        <i className='fa fa-eraser' />
+      </div>
+    );
+  }
+}
+
+ClearColumnButton.propTypes = {
+  onClick: PropTypes.func.isRequired,
+  intl: PropTypes.object.isRequired
+};
+
+export default injectIntl(ClearColumnButton);