…k form view

This commit updates the "hours planned" stat button, which can be be found in the task form view when the planning app has been installed.
Specifically, a new "hours to plan" stat button is added, which displays the hours that have been initially planned for the task completion minus the hours that have been forecasted from the planning app.
Clicking the button redirects the user to the planning app, but instead of the gantt to be filtered by the task, it is filtered by the assignee so that the remaining hours are planned.
If the task doesn't have an assignee, but it is generated through a sale order, the gantt will be filtered by the employees with the planning role indicated in the product form view.
Finally, if the task is not generated through a sale order, the button will just redirect to the unfiltered planning gantt.

The improvements that this task introduces aim at making planning more straightforward from the task view. Now, the user is able to see exactly the hours that still need to be planned, which ensures that no task is finished without all its allocated hours being planned.
Additionally, the redirection and filterring from the new stat button ensures a simple and intuitive flow for the user, who will be able to complete the task's planning without needing to navigate to the planning app from the home screen.

In order to implement these improvements, we had to modify the project_views.xml file as well as the project.py file in the project_forecast module.
In the former, we added code in order for the new stat button to be displayed next to the original "hours planned" one. In the latter, we added code to compute the remaining hours that have not been yet planned (in order for them to be displayed on the button) and code for executing the redirection action and chosing which filtering to apply.
We use two different ways to test the new improvements. First, we check the value reported on the stat button from the "test_remaining_forecast_hours()" test in the test_forecast.py file.
Second, we use a tour to test the flow of switching between the task view and the planning gantt view. We defined the tour inside project_planning_tour.js and created a new test file test_forecast_ui.py from which we execute and test the tour.