Purpose of this commit is to enhance quality of templates proposed by Odoo
and make them use notification layout when send by emails. Those emails
are cleaner and more up to date compared to other emails.
Including
* registration and reminder templates that are send directly by email
are improved to directly include the notification layout in the body
itself;
* improve layout of badge template;
* use notification layout when sending badge;
* improve layout of track confirmation template;
* use notification layout when sending templates based on stage change;
This commit is related to task ID 51122 (and PR #24052).