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/ui/components/column_header.js

@@ -0,0 +1,43 @@
+import React from 'react';
+import PropTypes from 'prop-types'
+
+class ColumnHeader extends React.PureComponent {
+
+  constructor (props, context) {
+    super(props, context);
+    this.handleClick = this.handleClick.bind(this);
+  }
+
+  handleClick () {
+    this.props.onClick();
+  }
+
+  render () {
+    const { type, active, hideOnMobile, columnHeaderId } = this.props;
+
+    let icon = '';
+
+    if (this.props.icon) {
+      icon = <i className={`fa fa-fw fa-${this.props.icon} column-header__icon`} />;
+    }
+
+    return (
+      <div role='button heading' tabIndex='0' className={`column-header ${active ? 'active' : ''} ${hideOnMobile ? 'hidden-on-mobile' : ''}`} onClick={this.handleClick} id={columnHeaderId || null}>
+        {icon}
+        {type}
+      </div>
+    );
+  }
+
+}
+
+ColumnHeader.propTypes = {
+  icon: PropTypes.string,
+  type: PropTypes.string,
+  active: PropTypes.bool,
+  onClick: PropTypes.func,
+  hideOnMobile: PropTypes.bool,
+  columnHeaderId: PropTypes.string
+};
+
+export default ColumnHeader;