Restrict Task Assignments to People Assigned to Project

[#1]

It's really easy to select an incorrect user to be assigned to a task. It'd be optimal to restrict the list of users you can assign to a task to those that are already tied to the project (either via sharing or through the staffing tab). It should be a setting to limit this per project (and on the template) as there are likely other types of projects where you'd want more flexibility in assignments.