[IMP] maintenance : add unscheduled maintenance

The feature added is a count of todo request that has no schedule date.
It will also add a link in the team dashboard that redirect to all
mainenance requests without a scheduled date.

The count is computed in the existing function '_compute_todo_requests'.
It will count requests that are not done and have no scheduled date.

This commit also modify the view and add the default filter 'unscheduled'
(created by a previous commit) for the action
'hr_equipment_todo_request_action_from_dashboard' which opens the maintenance
request for the active team.