Delete the old staffing code

[marcua]

There's a get new tasks button that is hidden by default on the Orchestra dashboard. It hooks into logic that automatically assigns you a new task. The work we've done with StaffBot and auto-staffing largely obviates the need for this button and the logic behind it. We should clean up that code once we ship v0 of autostaffing.

• settings.ORCHESTRA_ENABLE_NEW_TASK_BUTTONS
• enable_new_task_buttons
• enableNewTaskButtons
• newTasks
• preventNew (not just on backend but in both old and new Orchestra frontends)
• noTaskTimer
• newTaskError
• new_task_assignment (urls.py path: /orchestra/api/interface/)
• Exceptions these functions used to throw and were only used by these functions