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/reducers/alerts.js

@@ -0,0 +1,25 @@
+import {
+  ALERT_SHOW,
+  ALERT_DISMISS,
+  ALERT_CLEAR
+} from '../actions/alerts';
+import Immutable from 'immutable';
+
+const initialState = Immutable.List([]);
+
+export default function alerts(state = initialState, action) {
+  switch(action.type) {
+  case ALERT_SHOW:
+    return state.push(Immutable.Map({
+      key: state.size > 0 ? state.last().get('key') + 1 : 0,
+      title: action.title,
+      message: action.message
+    }));
+  case ALERT_DISMISS:
+    return state.filterNot(item => item.get('key') === action.alert.key);
+  case ALERT_CLEAR:
+    return state.clear();
+  default:
+    return state;
+  }
+};