What/Why

A Challenge Phase can have one and only one Evaluation Form associated. This means there should be a unique index on the evaluation_forms.phase_id. Users should not be able to select a Phase for a new or existing Form that already has a different Form associated with it.

Acceptance Criteria

• evaluation_forms table has a unique index on phase_id
• EvaluationForm model has a Rails validates :phase_id, :uniqueness
• The list of challenge phases on the Create/Edit Evaluation Form view should have phases with existing forms filtered out and not available for the user to select in the first place