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/actions/compose.js

@@ -16,6 +16,7 @@ export const COMPOSE_SUBMIT_FAIL     = 'COMPOSE_SUBMIT_FAIL';
 export const COMPOSE_REPLY           = 'COMPOSE_REPLY';
 export const COMPOSE_REPLY_CANCEL    = 'COMPOSE_REPLY_CANCEL';
 export const COMPOSE_MENTION         = 'COMPOSE_MENTION';
+export const COMPOSE_RESET           = 'COMPOSE_RESET';
 export const COMPOSE_UPLOAD_REQUEST  = 'COMPOSE_UPLOAD_REQUEST';
 export const COMPOSE_UPLOAD_SUCCESS  = 'COMPOSE_UPLOAD_SUCCESS';
 export const COMPOSE_UPLOAD_FAIL     = 'COMPOSE_UPLOAD_FAIL';
@@ -68,6 +69,12 @@ export function cancelReplyCompose() {
   };
 };
 
+export function resetCompose() {
+  return {
+    type: COMPOSE_RESET,
+  };
+};
+
 export function mentionCompose(account, router) {
   return (dispatch, getState) => {
     dispatch({