[IMP] (website_)mail: improve routes and management of mail followers

Purpose of this commit is to improve model of followers, notably management
code and its use in routes. Indeed it is quite an old model and code had
to be cleaned a bit to improve code readability and maintenance.

In this commit we

  * remove unnecessary code examples in gamification about followers: using
    that model as example of code for goals is probably not a good idea as it
    is technical;
  * rewrite routes called by JS are simplified to better match JS
    implementation;
  * introduce computed fields to fetch related partner or channel name,
    email (partner only) and active status;

LINKS

Task 1933771
Task 2078313

closes odoo/odoo#39808

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Co-authored-by: Remy Voet <ryv@odoo.com>
Co-authored-by: jgi-odoo <jgi@odoo.com>
Co-authored-by: Xavier-Do <xdo@odoo.com>