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/actions/alerts.js

@@ -0,0 +1,24 @@
+export const ALERT_SHOW    = 'ALERT_SHOW';
+export const ALERT_DISMISS = 'ALERT_DISMISS';
+export const ALERT_CLEAR   = 'ALERT_CLEAR';
+
+export function dismissAlert(alert) {
+  return {
+    type: ALERT_DISMISS,
+    alert
+  };
+};
+
+export function clearAlert() {
+  return {
+    type: ALERT_CLEAR
+  };
+};
+
+export function showAlert(title, message) {
+  return {
+    type: ALERT_SHOW,
+    title,
+    message
+  };
+};