✨ Add "back-to-start" custom event

✨ Add custom event when we click on next or previous button

🐛 Fix bugs:

- Fix the empty end date bug
- align the next button & the previous button
- hide the next button if the event is the last
- hide the previous button if the event is the first
- show the next button and the previous button if not
- update the text of the next button & of the previous button when the
  event selected is updated

✨ Add a home button to go to the start of the story
✨ Add the infini symbol if the event is ongoing