Implement hotkeys for web UI (#5164)

* Fix #2102 - Implement hotkeys

Hotkeys on status list:

- r to reply
- m to mention author
- f to favourite
- b to boost
- enter to open status
- p to open author's profile
- up or k to move up in the list
- down or j to move down in the list
- 1-9 to focus a status in one of the columns
- n to focus the compose textarea
- alt+n to start a brand new toot
- backspace to navigate back

* Add navigational hotkeys

The key g followed by:

- s: start
- h: home
- n: notifications
- l: local timeline
- t: federated timeline
- f: favourites
- u: own profile
- p: pinned toots
- b: blocked users
- m: muted users

* Add hotkey for focusing search, make escape un-focus compose/search

* Fix focusing notifications column, fix hotkeys in compose textarea

app/javascript/mastodon/features/notifications/containers/notification_container.js

@@ -1,6 +1,7 @@
 import { connect } from 'react-redux';
 import { makeGetNotification } from '../../../selectors';
 import Notification from '../components/notification';
+import { mentionCompose } from '../../../actions/compose';
 
 const makeMapStateToProps = () => {
   const getNotification = makeGetNotification();
@@ -12,4 +13,10 @@ const makeMapStateToProps = () => {
   return mapStateToProps;
 };
 
-export default connect(makeMapStateToProps)(Notification);
+const mapDispatchToProps = dispatch => ({
+  onMention: (account, router) => {
+    dispatch(mentionCompose(account, router));
+  },
+});
+
+export default connect(makeMapStateToProps, mapDispatchToProps)(Notification);