[FIX] project: duplicate project must copy followers

Since duplicating a project copies the project
tasks, with the followers, the project followers must
remain as well.

This is done in the already overriden method
`copy` instead of simply adding `copy=True`
on the according one2many field, since this field
is defined on the inherited model
`mail.thread`, and to add `copy=True`, in the old api,
we would have to re-define the `message_follower_ids`
field within the `project.project`
model completely, making sure to copy all properties

opw-680313