[MERGE][IMP] mail, various: add automatic activities generation to ease business flows

This merge make several application automatically generate activities based
on business requirements in order to ease the daily job of people using it.

In this merge

 * methods allowing to create, reschedule, mark as done or unlink activites
   are added in mail.activity.mixin;
 * automated activities are flagged using the newly-added automated field
   to distinguish them from other activities;
 * mail.activity.mixin is added to some models to ensure activities are
   activated;
 * several main applications use this API to integrate activities automation
   in their flow: hr_holidays (leave and allocation), hr_expense (expense
   report), fleet (vehicle contract), maintenance (maintenance request),
   sale (sale order);

Please see sub-commits for more details about the mixin update as well as
the integration in the various applications.

This merge is linked to task ID 39578. Closes #20799 .

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