[MERGE][IMP] digest, various: improve digest content and tips

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 TEMPLATE

Improve template according to FP specifications, aka

    Header
    Up to 2 tips
    KPIs
    Want to customize?
    Mobile Tip
    Footer

SPECIFICATIONS: QWEB TEMPLATE

Digest now uses 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.

SPECIFICATIONS: DELAY BEFORE SENDING

In this commit we introduce a way to set a delay before sending a tip.
Using a new field, a tip can be send a few days after its module installation.
This require to store the source module on the tip itself. When installed
from data it will be automatically filled using its xml_id if not set

SPECIFICATIONS: TIPS

Improve tips content and display according to FP specifications.

LINKS

Task ID 2197417
Community PR odoo/odoo#51619
Enterprise PR odoo/enterprise#10697
Upgrade PR odoo/upgrade#1256

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>