Add field settings to enforce uniqueness of keys

Problem/Motivation

Sometimes you want the keys to be unique.
It would be great if we added a field-setting that had three options as follows:

• not-unique - current functionality
• unique per bundle - enforce that the key only exists once for a given bundle
• globally unique - enforce that the key is unique to all bundles

Proposed resolution

Add the new feature and associated tests

Remaining tasks

The lot

User interface changes

API changes

Data model changes