Purpose of this commit is to clean and ease the use of tracks in
event management.
* status is change to stages. Those are global for all event as we
consider track acceptation process as similar across events;
* change speakers many2many to a one2many. Most event have only one
speaker or at least a main speaker;
* add various chatter features: tracking of template, subtypes,
suggested recipients, ... Also add activities on tracks and their
various filters;
* add automatic mailing on stages like already done in tasks or
issues;
* tweak display of tracks on agenda. Event manager see unpublished
tracks with the right label in order to know the status of the various
event tracks.
Mitali Patel