[IMP] mail: add activity_search method in mail.activity.mixin

PURPOSE

After this commit it will be easier to find all the corresponding automated
activities in a model.

SPECIFICATIONS

Add an activity_search method in the mail.activity.mixin model to ease
the searching on mail.activity corresponding to the model. It is already
inlined in activity_reschedule and activity_feedback.

LINKS

Task ID : 2127364
PR : #40708