Fixes some editclass logic.

 * The user isn't necessarily a teacher of the class (might be an admin),
and the user doesn't need to be added as a teacher (the user is either a
current teacher or an admin). So have ClassCreationController.editclass()
take a current_user argument, rather than a user argument.

 * Do not call ClassCreationController.force_availability(). The editing user
is either an admin (whose availability is irrelivant / shouldn't be edited),
or a teacher who (in theory) already indicated availability.

 * Call ClassCreationController.require_teacher_has_time() for all teachers
of the class, rather than the current user (who might not be a teacher)
and only the current user.

 * Allows ClassCreationController.make_class_happen() to be called without
a user, since we don't need to associate a user with the class if the user
is already a teacher of the class.

 * Have ClassCreationController.require_teacher_has_time() take a current_user
argument, so that different messages can be displayed depending on
whether the teacher violoating the time constraint is the current user or
a different teacher.
Merge pull request #742 from learning-unlimited/old-editclass-fixes

Fixes some editclass logic.