Currently the Document version of an issue (JRA-6874) is limited to system fields, and is badly hard-coded. We need to introduce a new interface for fields that can load their values from the document, and use that instead.

We can then allow custom fields to load their own values from the document. This would improve performance in the issue navigator significantly when large numbers of custom fields are displayed (from my calculations - it would roughly halve the time to display the table).