Ticket #2162: Allow team leaders to withdraw people

Except blueshirts and themselves.
Clarify the withdrawn user warning and make it more obvious.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
Fix #2162: Make it possible for team leaders to withdraw students.

This merges Krenair's branch 'ticket2162' which I extended with some
minor UI changes to make the warnings a little clearer.

This introduces a new button on the edit view which lets team leaders
withdraw their own students. At the moment, being in the withdrawn group
doesn't actually do anything, but eventually will probably be used to
prevent access to our systems, so we warn the team leader that that's
what happens.