Revert "Override the otree.db.models.CharField.formfield method in order to use the floppyforms widget."

This reverts commit 5ef5a089834ca65da5c99a770c462b4331792c56.
Adding simple testsuite to check if floppyform fields are used for custom DB fields.
Using custom formfield_callback to use floppyform fields for all relevant DB fields.
Using floppyforms fields for RandomCharField.
Using our own version of a modelform_factory in views so that the custom formfield_callback is taken into account.
Adding MoneyField with MoneyInput as widget.