BUG Fixes #2682 - Regression in #2595

This fixes a slight regression in PR #2595 which introduced the ability to sort on relation fields. The side effect of this is that it assumed that any `Field.SubField` composite field in `summary_field` had to be a foreign relation, when in fact it could be a simple method call on a direct DBField object.

This PR relaxes the constraints, where relations which would previously cause a crash, now simply translate into non-sortable columns, as was the previous behaviour.