User list for admins.

Users index and show actions.  Only admin users may view the user list
(due to privacy regulations regarding students.)  Admins can delete users
from this page.
Controller tests for users index.

Confirm that index is only visible to admins.
Allow users to see their own user page.

The `:require_user_login_or_admin` filter is a huge hack -- need to
figure out the proper way to do this.
Further tests for UsersController#show.

Admins have full access, logged-out users have no access.
Refactor user controller specs.

Create `login_as` controller macro.  Change .new + .save! to .create.