A base add form for content rule.
Use this for rule elements that require configuration before being added to
a rule. Element types that do not should use NullAddForm instead.
Sub-classes should define create() and set the form_fields class variable.
Notice the suble difference between AddForm and NullAddform in that the
create template method for AddForm takes as a parameter a dict 'data':
def create(self, data):
whereas the NullAddForm has no data parameter:
Definition at line 23 of file formhelper.py.