Use form validation instead of triggering integrity error

[munnsmunns]

Create a generic helper method to check unique by assessment validation, when we don't include the assessment in a model form. This helper method is applied to numerous forms via a clean_FOO method.

We initially tried including the assessment field in the form as a disabled hidden field, but ran into some issues with the value being hydrated correctly with a foreign key relation on updates.

• assessment.DatasetForm
• lit.SearchForm
• riskofbias.RoBDomainForm
• summary.SummaryTableForm
• summary.VisualForm
• summary.DataPivotForm

In addition, expose non_field_errors in the react DjangoForm if errors are returned in validation.

[shapiromatron]

This isn't working for me; when I try to update an existing summary.Visual without changing anything, I get a field form error.

I have a few ideas, I'm going to start working on this...

[shapiromatron]

@munnsmunns ok, made some edits and I think this is a reasonable solution that works well - as a bonus - net negative code! Please give it a review to makes sure it works as expected for creates, updates, etc.