[ADD] Add the website_links module.

This module tracks clicks in mass mailing mails and allow the generation of trackable links in a website interface.

Modules modifications
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Refractoring of the crm_tracking_* classes in a new module.

* Extract the crm_tracking_* from the crm module into a new module "utm"
* Remove the crm_mass_mailing bridge module
* New dependencies of mass_mailing and website_links to utm.