This commit changes some of the way that users will work with subtasks in the project app. Initially, it changes subtasks from a many2many to a one2many field. We also add the ability for subtasks to be viewed straight from the kanban view by drawing a list of them inside of the kanban box of the parent task. task-3085016 closes odoo/odoo#112279 Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>