[REF][FIX] link_tracker, mass_mailing: factorize method creating link tracker click values

This commit factorizes the link.tracker.click values computation out of
the add_click method. This method is called when customers click on links
in email that are tracked in Odoo.

Purpose of this commit is

 * to effectively put mass mailing related code into mass mailing.
   Currently link_tracker holds code related to mass mailing which is
   not in its dependency tree. Let us therefore try to be a bit modular
   and add a proper inheritance in mass mailing;
 * allow addons to inherit from the click values prepare method to add
   specific values depending on their behavior like mass mailing adding
   links to a mass mailing and a mass mailing campaign;

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