[ADD] add `project_assignees` add-on

Add a field in ``project.task`` to assign users to task. This
completes ``user_id`` field, that is then considered as the owner of
the task. Assignment becomes optional and can now target multiple
users.

Task: JOI-13
Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

project_assignees/models/__init__.py

@@ -0,0 +1,2 @@
+
+from . import project_task