This commit adds a new digest module allowing to send recurrent digests by
email. Those contain a summary of the activity and display various KPIs
allowing to see the activity at a glance. It is also a great tool to improve
user engagement.

Digest module contain the core implementation of digests emails as well as
first basic KPIs. Future commits will gradually add KPIs in various main
addons.

Digests consist in email templates that have to call some methods on digest
module (compute_kpis, compute_tips, compute_kpis_actions) and render it
accordingly. A default digest is given in data allowing to tune it or to
duplicate and modify it.

Frequency is configurable on the digest; digests can be weekly, monthly or
send every 3 months. New employee users are added to the default widget.
KPIs are computed for each subscribed users so that results match its
groups and related access rights. Default digest therefore work for all users
as displayed results depend on their right without having to define separate
digests just for that purpose.

Implementation is done with Studio in mind. KPIs are computed fields on the
digest model. It means using Studio custom computed fields can be defined
to add customization to the digests, using fields named x_kpi_... .

Digests also holds tips to remind and warn users of features to activate
or configure like mailgateway.

KPIs can be linked to actions so that the section contain a link allowing
to directly jump on a given menu, allowing to directly link reporting
menus for example.

This commit is related Task ID 30655 and PR #18318.

Co-authored-by: Siddharth Gajjar <sga@odoo.com>