Improved tracking value change

This module makes it easier to see what changed on records and when. It adds a new menu entry called "Improved Tracking Values" under the Email/Discuss menu, opening a list of tracked changes sorted from the most recent. Each line shows the date, the record concerned, the field that changed, the old value and the new value, and the model. A search view lets you filter by model and group changes by model, and a form view shows the same information for a single change.

Behind the scenes, it adds readable formatted values for tracked fields, so numbers, dates, amounts and text are displayed as plain values instead of raw technical data. It also extends change tracking to many-to-many and one-to-many fields: when such a field changes, the names of the related records are stored and shown, and the list of record identifiers is kept as well.

It touches the existing tracking and message models (mail.tracking.value and mail.message) and adds related fields for the record name and model. No new business rules, emails or scheduled jobs are introduced; the module is purely about making tracked changes easier to consult.