We were lacking tests for most datagrid columns, and this meant that we

didn't catch a series of implementation issues, such as a mixture of

safe and non-safe strings for HTML, non-string return types for

rendering (integers and a Repository instance in one case).
Upcoming work will be addressing these and adding support required for

a dashboard API resource. To prepare for that, and to provide a good

base for upcoming fixes, this change fleshes out our datagrid column

test suite to cover all columns but the Repository column (which can't

easily be tested without those fixes).

Unit tests pass.