Products.CMFActivity.ActivityTool: Store user object in activity.

When spawning an activity, store the current security context's user in
the Message object itself, so the activity security context can be
re-created with the same security during activity execution.
This allows a user to be modified (different groups, global roles, maybe
removed altogether) after they spawned activities and before these activities
could run.
It also means that any temporary custom group or global role granted to
that user (by a privilege elevation mechanism out of the scope of this
change) will still be effective during the activity execution.
This follows the principle that
  foo.activate(...).bar(...)
should be equivalent to its "immediate execution" version
  foo.bar(...)
by ensuring that the security context of the activity is the same as the
one which was applied to the code which spawned that activity,
independently of any intermediate configuration change - hence improving
(deferred and fragmentary) transaction isolation.

This also removes the need to look the user up, then looking up their
assignments (and other documents involved in group computation), etc,
saving the cost of these calls.

Also, remove redundant user_name argument of Message.changeUser method.