User-customizable display columns for cohort summary info

[Viqsi]

From Veronica's todo document:

• Allow users to customize display columns for summary info on cohorts

To call this "tricky" would be a bit of an understatement. It would require significant work on every single level of the application, because the columns are determined by a backend query. It may be too much work for too little return. But it's been suggested, so we're noting it here for the time being even if it never gets done.