Add special handling for text fields with the CLEANHTML type. This should
be used when students and teachers can edit the same field (you can't trust those students).
Applies cleaning on submitted data, and on data stored in the DB before it is put back in an editing form.
Helper methods (ie custom methods on a persistent) should not be callable via set() or get() even
if they start with set_ or get_. They are not the same as custom getters and setters for real persistent
properties.
