[autoform] Make RelationField.fromcardinality a class method factory
Instead of having the "RelationField" class hard-coded in this factory
method (thus preventing any subclass to properly use it), we make that
method a classmethod and instantiate the actual class instead of the
base one.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
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