[IMP][ADD] crm: add the concept of activities, allowing

to define custom light workflows on opportunities.

Activities are linked to mail.message.subtype through inheritance.
This means that the follow mechanism works for activities. When
an activity is done, a message with the matching subtype is posted
on the opportunity.

Activities are internal by default. They are only visible by
employees.

A shortcut in the 'log a note' chatter option allows to post a
note linked to an activity (subtype).