Ensure the NullUser can't withdraw students.

Change-Id: Ia879c5bbe8476d217ca55a560a65d6f6b8839201
Ensure non-authenticated users can't withdraw users either.

Change-Id: I823e51ad2da5d6ac9a1f366c12d287958ff8188b
Add section names in tests.

Change-Id: Ib0b6ab68dc6eb6182f963c0d565ffec8526d1fa2
Add a __repr__ to User for nicer debugging.

Change-Id: I759ff42ea4f9e892a28d88eeaed5ac6d6d6c5e0b
Add a can_view method to User.

This is, for the moment, a synonym for can_administrate, but will
be extended to allow for some special access cases.

Change-Id: I3bee9e3f9a1adec08268974f7d9e320883d34351
Let some blueshirts see (limited) details of all competitors

This adds support for a 'mentors-extra' group of blueshirts who
have access to see most of the details for any other user. This
access includes their full name, but does not include their email
address.

The primary use case for this is the reception desk at the competition,
where we'll need to check the details against the people that turn
up, and (in particular) their media consent status.

Change-Id: I8175a526601ef2e5638f12aa9d47016a1acb4be0