This commit improve the tree and form view of gamification.karma.rank
and correct a bug on auto generated email when a user pass to the
next rank.
- Add a stat button in form view and column in tree view to know
how many users have the current rank
- Split Description and Motivational in 2 pages
- Hide the line that mention the next rank the user can get when
there is no more rank to get in email templates
- Add a menu to access badges tree view
Part of https://github.com/odoo/odoo/pull/46634
task-2201708