[IMP] crm: added reason for loosing opportunities
[IMP] sale & sale_stock : removed 'Enable Invoicing Sales order lines' group from user setting and 'Generate invoices based on the sales order lines' group from sale setting.
[IMP] website_event: when buying tickets, display them by type
[CLEAN] event: removed unnecessary count_registrations field;
on form view, display attendees (not canceled, ...) instead of
registration count.
[IMP] event: chatter on registration proposes to add
the partner and attendee email as followers.
[REF] mail: _track update.

_track is not a method that returns the subtype to trigger in a given
tracking context. A tracking will now lead to only one subtype
being trigerred, instead of having multiple possible subtypes.

Now, one tracking leads to one message with zero or one subtype.
This simplifies the model for future evolution of chatter and mail.