[REF] digest: move template to QWeb

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

Digest now use qweb views instead of a standard mail.template record for
rendering as

  * template is very custom;
  * probability of breaking it while edition the mail template is high;
    it is easier to maintain and extend in qweb;
  * only body was really used, other mail-related fields were not used;

We therefore remove the mail template data and the field used for it on
the digest model. Template is now forced to a Qweb view.

We can also remove the template_id field. Indeed we think that having
different templates for digests is a really advanced use case we do not
want to support.

Finally kpis and action computation is rewritten. It now returns an unique
structure containing all information in a more neutral way, holding values
for 3 columns. Purpose is to be less date-oriented in data construct and
allow people to use digest qweb template to display a 3-columns KPIs content
even if not related to yesterday / last 7 days / last 30 days.

LINKS

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