Small fix to the parent fragment update

Implemented a first version of the to_string method for JSON fragment

Fixed ethical_issue name in structured answer

Removed schema presence validation for now

Updated fragment models according to data model

Fixed project fragment generation when creating a plan

References to children fragment should be updated when creating or deleting a child fragment

STI column value should be more readable

Updated StructuredAnswer model with new columns

Removed parent_id column

(WIP) Implemented structured schema into rails models

Fix on migration

Added structured schema/answers models and tables