_track is now 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 one subtype, or without subtype
if there is none matching. This simplifies the model for future evolution of
chatter and mail.
[IMP] project, project_issue: cleaned task and issue subtypes, now having
opened for created / assigned.
Thibault Delavallée
d4e9640e34