[IMP] survey: improve session "short link" management

PURPOSE

Improve the various sessions flow to make it easier for attendees to reach the
session and to allow better interactions between the host and his audience.

This comes with small usability improvements as well as increased
"beautifulness" by using custom layouts & animations.

SPECS

- For live session only:
  Currently, the "short link" feature is good but it still needs to be improved
  with the following points:
    - Make the default "code" shorter (4) and digit only.
    - Allow to customize the "code"
    - Check for potential collisions

Added at the same time:
- input element is focused when you land on the '/s' page.

LINKS

PR #46768
Task 2208574