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/relative_timestamp.js

@@ -0,0 +1,20 @@
+import React from 'react';
+import { injectIntl, FormattedRelative } from 'react-intl';
+import PropTypes from 'prop-types';
+
+const RelativeTimestamp = ({ intl, timestamp }) => {
+  const date = new Date(timestamp);
+
+  return (
+    <time dateTime={timestamp} title={intl.formatDate(date, { hour12: false, year: 'numeric', month: 'short', day: '2-digit', hour: '2-digit', minute: '2-digit' })}>
+      <FormattedRelative value={date} />
+    </time>
+  );
+};
+
+RelativeTimestamp.propTypes = {
+  intl: PropTypes.object.isRequired,
+  timestamp: PropTypes.string.isRequired
+};
+
+export default injectIntl(RelativeTimestamp);