Clarify the withdrawn user warning and make it more obvious.

Even knowing it was there, I managed to look past it!

Change-Id: Iea0bb891130eed4e971e07aafb426fd3d805e89f
Make it clearer that withdrawing users is one-way.

Add a title to the button, and require a confirmation before actually
doing anything. Have the confirmation box explain the consequences
of withdrawing the student, including the need to contact us to undo it.

Change-Id: Idca81b2a1bd2986ca9704d79155c0c39dd710a31