PURPOSE
Gamification display is slow when being heavily used. Indeed the m2m towards
users is used in kanban and form views, leading to a huge data fetch when
being used in production environment with lots of users.
SPECIFICATIONS
Remove user_ids m2m from views. Indeed in some heavily used DBs you may have
more than 10K records in user_ids, leading to a huge m2m tags to display.
Replace it by an optimized count, used in
* form view: as a stat button redirecting to participants of a challenge;
* kanban view: a line (replacing the 11 first avatars) redirecting to
participants of a challenge;
Task-2678295
Part-of: odoo/odoo#79599
Thibault Delavallée
07c872d674