[nostory] fix destroy dependency btw/ group & custom from
[nostory] fix destroy dependency btw/ group & custom from

* BUG: active record throws error when deleting a group that has a
  custom form
* CAUSE: we violate a foreign key constraint that by not deleting the
  custom form when we delete the group
* FIX: add a `dependent: :destroy` helper to delete associated custom
  forms when we delete a group

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