[16.0][ADD] mail_message_view: list the chatter messages in a menu

[smorita7749]

This module lists the chatter messages in a dedicated Discuss > Message History
menu, where they can be searched, filtered and grouped.

The chatter mixes the messages posted by users with the ones logged by the system
(field tracking, automated notifications), which makes a long history tedious to
follow. On top of that, the tracked field changes are only readable from the
chatter, as tracking_value_ids is restricted to base.group_system.

The list therefore shows, as plain text, a preview of the message body and a
summary of the tracked field changes, e.g. Status: RFQ -> Purchase Order, and
provides filters telling the messages posted by users apart from the ones logged
by the system.

Notes:

• the views are variants of the standard message views (inherited in primary
  mode), so the standard Messages list is left unchanged;
• a button opening the list for a single record is added on the form views of the
  models listed in the mail_message_view.button_models system parameter;
• the menu is restricted to a dedicated group, as listing the messages of every
  document is an expensive query.

Targeting 16.0 as this is the series where the module is used, but I am happy to
forward-port it first if maintainers prefer new modules to land on the latest
branch.

This is a new module of about 1000 lines, which is above the 500 lines of the OCA
metrics red lines. Please let me know how you would like to proceed with the
review.

Assisted-by: Claude Opus 5

@qrtl QT7086