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/styles/basics.scss

@@ -94,9 +94,12 @@ button {
 }
 
 .app-holder {
-  display: flex;
-  width: 100%;
-  height: 100%;
-  align-items: center;
-  justify-content: center;
+  &,
+  & > div {
+    display: flex;
+    width: 100%;
+    height: 100%;
+    align-items: center;
+    justify-content: center;
+  }
 }