Add column "Price" to project and task grid

This column should contain the final price of a task.

For a project it contains the sum of the final prices of all tasks the project contains.

The field should be updated each time an analysis of a task has been re-run and every time, the price scheme of a task is changed.

It must be ensured by the development done in

https://jira.translate5.net/browse/TRANSLATE-3483

that the price fields can only be seen by PM users.