Implement tag auto-completion by history (#6621)

This is a functionality similar to one implemented in Pawoo:
21a3c70f80

app/javascript/mastodon/settings.js

@@ -44,3 +44,4 @@ export default class Settings {
 }
 
 export const pushNotificationsSetting = new Settings('mastodon_push_notification_data');
+export const tagHistory = new Settings('mastodon_tag_history');