Add a confirmation on deleting an assignment on the admin screen

[ghost]

Currently when admin/instructor tries to delete an assignment, there is no alert/confirmation shown. This is risky. We need to add a confirmation step to avoid losing data.

[efg]

If any reviews have been written, the instructor is asked to confirm. Even if not, the system should still ask the instructor. Not clear if this happens.

[yangsong8]

Part of E1662 in fall 2016, rejected.

[ferryxo]

Using the production system, I tried to create an assignment and delete it directly. It prompted for a confirmation.