Track frequently used emojis in web UI (#5275)

* Track frequently used emojis in web UI

* Persist emoji usage, but debounce commits to the settings API

* Fix #5144 - Add tooltips to picker

* Display only 2 lines of frequently used emojis

app/javascript/mastodon/actions/emojis.js

@@ -0,0 +1,14 @@
+import { saveSettings } from './settings';
+
+export const EMOJI_USE = 'EMOJI_USE';
+
+export function useEmoji(emoji) {
+  return dispatch => {
+    dispatch({
+      type: EMOJI_USE,
+      emoji,
+    });
+
+    dispatch(saveSettings());
+  };
+};