Add ability to specify field constraints

[chris48s]

Overview

It would be useful if datapackage-ui exposed the constraints concept from the tableschema spec
https://specs.frictionlessdata.io/table-schema/#constraints

At a code level, this would require us to

• Add a ConstraintField component. Each instance of EditorField my optionally have a ConstraintField.
• One level down we would then need corresponding UPDATE_CONSTRAINT and REMOVE_CONSTRAINT reducers.

AFAICT as long as we build up the schema correctly on input datapackage-js should handle the additional validation for us.

The bigger challenge is probably how to present it visually. EditorField is already a bit squished with 4 UI elements and supporting constraint fully requires an additional 8 interface elements

(Feature request arising from the eLife Innovation Sprint 2020)

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