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;
Thibault Delavallée
7accd77999