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

package.json

@@ -80,6 +80,7 @@
     "rails-ujs": "^5.1.2",
     "react": "^16.0.0",
     "react-dom": "^16.0.0",
+    "react-hotkeys": "^0.10.0",
     "react-immutable-proptypes": "^2.1.0",
     "react-immutable-pure-component": "^1.0.0",
     "react-intl": "^2.4.0",