[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/project_task.py

@@ -0,0 +1,10 @@
+
+from odoo import models, fields, _
+
+
+class Task(models.Model):
+    _inherit = "project.task"
+
+    user_id = fields.Many2one(string=_("Owner"))
+    assignee_ids = fields.Many2many('res.users', 'assignee_ids_rel', string='Assignees')
+