[REF] digest: make digest template bioutifoul

PURPOSE

Make digest email and tips more appealing. The goals of these tips are

  * to encourage the adoption of other apps (Did you know ?);
  * to make Odoo look more fun (Fun tips and tricks, young and dynamic style);
  * to show social proof and increase trust (emphasis on already existing
    projects / customers to);

SPECIFICATIONS

Improve template according to FP specifications, aka
    Header
    Tip
    KPIs
    Preference text
    Want to customize?
    Mobile Tip
    Footer

LINKS

Task ID 2197417
PR odoo/odoo#51619
Upgrade PR odoo/upgrade#1256

Co-Authored-By: Elisabeth Dickinson <edi@odoo.com>
Co-Authored-By: Thibault Delavallée <tde@odoo.com>